First of all, ensure you don't have any deprecated!

The Symfony documentation explains it well, but let's sum up:

• install the phpunit bridge ($ composer require --dev symfony/phpunit-bridge)
• also check all your pages using web profiler and be ensure there is no deprecation error handled
• found errors and need help about how to fix it ? I did a sort of guide.
• if you start from Symfony 2.7 LTS, fix deprecations using Upgrade information 2.7 => 2.8

You need to update symfony/symfony metapackage to 3.x branch as described by official docs:

{
    "...": "...",

    "require": {
        "symfony/symfony": "3.0.*",
    },
    "...": "..."
}

But, it's not enough because you also need to update the "autoload", "autoload-dev" and "extra" sections. The reasons are the following ones (afaik):

• Appkernel and AppCache classes are autoloaded by Composer
• Your application tests are localized in a new tests folder
• var folder contains cache and log folders

You also need to remove bin-dir property from config section.

You can update your dependencies using composer:

$ composer update symfony/symfony --with-dependencies --no-scripts

I use --no-scripts option because otherwise installation scripts will fail.

• remove app/bootstrap.php.cache and app/autoload.php files
• app/Kernel.php should be updated in order to set the new location of cache and logs path
• assetic section from app/config.yml and app/config_dev.yml should be removed (or the AsseticBundle re added)
• the _webconfigurator route MUST be removed from app/config/routing_dev.yml as this doesn't exists anymore (since v4 of DistributionBundle?)
• move app/console to bin/console and update it
• move all tests from src/*Bundle/Tests/* to tests/*Bundle/* (and update namespaces)
• update app.php, app_dev.php and app_test.php to update app/bootstrap.php.cache to var/bootstrap.php.cache
• move app/SymfonyRequirements.php to var/SymfonyRequirements.php
• move phpunit.xml.dist to root folder (so we don't need to call phpunit -c app/ anymore !)
• update .gitignore file using Symfony3 one

In Symfony 3, the framework session configuration is already setup:

framework:
# [...]
    session:
        # http://symfony.com/doc/current/reference/configuration/framework.html#handler-id
        handler_id:  session.handler.native_file
        save_path:   "%kernel.root_dir%/../var/sessions/%kernel.environment%"

If you decide to report this change, don't forget to create the sessions folder in var.

As there is only one folder that contains all the tests, we can simplify the configuration:

// before
<directory>../src/*Bundle/Tests</directory>
<directory>../src/*/*Bundle/Tests</directory>
<directory>../src/*/Bundle/*Bundle/Tests</directory>

// after
<directory>tests</directory>

Don't forget to declare the path to the application, this is required:

// before
<!--
<php>
    <server name="KERNEL_DIR" value="/path/to/your/app/" />
</php>
-->

// after
<php>
    <server name="KERNEL_DIR" value="app/" />
</php>

And update the path to the autoloader:

// before
bootstrap                   = "autoload.php"

// after
bootstrap                   = "vendor/autoload.php"

Then execute post-install-cmd composer commands to generate the bootstrap.php.cache file:

composer run-script post-install-cmd

You can launch tests:

./vendor/bin/phpunit

Composer installation can be completed:

$ composer install && composer dumpautoload -o

• The intention option was deprecated and will be removed in 3.0 in favor of the new csrf_token_id option.

• The csrf_provider option was deprecated and will be removed in 3.0 in favor of the new csrf_token_manager option.

• The "cascade_validation" option was deprecated. Use the "constraints" option together with the Valid constraint instead. Contrary to "cascade_validation", "constraints" must be set on the respective child forms, not the parent form.

  Before:

  $form = $this->createFormBuilder($article, array('cascade_validation' => true))
      ->add('author', new AuthorType())
      ->getForm();

  After:

  use Symfony\Component\Validator\Constraints\Valid;
  
  $form = $this->createFormBuilder($article)
      ->add('author', new AuthorType(), array(
          'constraints' => new Valid(),
      ))
      ->getForm();

  Alternatively, you can set the Valid constraint in the model itself:

  use Symfony\Component\Validator\Constraints as Assert;
  
  class Article
  {
      /**
       * @Assert\Valid
       */
      private $author;
  }

• Type names were deprecated and will be removed in Symfony 3.0. Instead of referencing types by name, you should reference them by their fully-qualified class name (FQCN) instead. With PHP 5.5 or later, you can use the "class" constant for that:

  Before:

  $form = $this->createFormBuilder()
      ->add('name', 'text')
      ->add('age', 'integer')
      ->getForm();

  After:

  use Symfony\Component\Form\Extension\Core\Type\IntegerType;
  use Symfony\Component\Form\Extension\Core\Type\TextType;
  
  $form = $this->createFormBuilder()
      ->add('name', TextType::class)
      ->add('age', IntegerType::class)
      ->getForm();

  As a further consequence, the method FormTypeInterface::getName() was deprecated and will be removed in Symfony 3.0. You should remove this method from your form types.

  If you want to customize the block prefix of a type in Twig, you should now implement FormTypeInterface::getBlockPrefix() instead:

  Before:

  class UserProfileType extends AbstractType
  {
      public function getName()
      {
          return 'profile';
      }
  }

  After:

  class UserProfileType extends AbstractType
  {
      public function getBlockPrefix()
      {
          return 'profile';
      }
  }

  If you don't customize getBlockPrefix(), it defaults to the class name without "Type" suffix in underscore notation (here: "user_profile").

  If you want to create types that are compatible with Symfony 2.3 up to 2.8 and don't trigger deprecation errors, implement both getName() and getBlockPrefix():

  class ProfileType extends AbstractType
  {
      public function getName()
      {
          return $this->getBlockPrefix();
      }
  
      public function getBlockPrefix()
      {
          return 'profile';
      }
  }

  If you define your form types in the Dependency Injection configuration, you should further remove the "alias" attribute:

  Before:

  <service id="my.type" class="Vendor\Type\MyType">
      <tag name="form.type" alias="mytype" />
  </service>

  After:

  <service id="my.type" class="Vendor\Type\MyType">
      <tag name="form.type" />
  </service>

  Type extension should return the fully-qualified class name of the extended type from FormTypeExtensionInterface::getExtendedType() now.

  Before:

  class MyTypeExtension extends AbstractTypeExtension
  {
      public function getExtendedType()
      {
          return 'form';
      }
  }

  After:

  use Symfony\Component\Form\Extension\Core\Type\FormType;
  
  class MyTypeExtension extends AbstractTypeExtension
  {
      public function getExtendedType()
      {
          return FormType::class;
      }
  }

  If your extension has to be compatible with Symfony 2.3-2.8, use the following statement:

  use Symfony\Component\Form\AbstractType;
  use Symfony\Component\Form\Extension\Core\Type\FormType;
  
  class MyTypeExtension extends AbstractTypeExtension
  {
      public function getExtendedType()
      {
          return method_exists(AbstractType::class, 'getBlockPrefix') ? FormType::class : 'form';
      }
  }

• In Symfony 2.7 a small BC break was introduced with the new choices_as_values option. In order to have the choice values populated to the html value attribute you had to define the choice_value option. This is now not any more needed.

  Before:

  $form->add('status', 'choice', array(
      'choices' => array(
          'Enabled' => Status::ENABLED,
          'Disabled' => Status::DISABLED,
          'Ignored' => Status::IGNORED,
      ),
      'choices_as_values' => true,
      // important if you rely on your option value attribute (e.g. for JavaScript)
      // this will keep the same functionality as before
      'choice_value' => function ($choice) {
          return $choice;
      },
  ));

  After (Symfony 2.8+):

  $form->add('status', ChoiceType::class, array(
      'choices' => array(
          'Enabled' => Status::ENABLED,
          'Disabled' => Status::DISABLED,
          'Ignored' => Status::IGNORED,
      ),
      'choices_as_values' => true
  ));

• Returning type instances from FormTypeInterface::getParent() is deprecated and will not be supported anymore in Symfony 3.0. Return the fully-qualified class name of the parent type class instead.

  Before:

  class MyType
  {
      public function getParent()
      {
          return new ParentType();
      }
  }

  After:

  class MyType
  {
      public function getParent()
      {
          return ParentType::class;
      }
  }

• The option "options" of the CollectionType has been renamed to "entry_options". The usage of the option "options" is deprecated and will be removed in Symfony 3.0.

• The option "type" of the CollectionType has been renamed to "entry_type". The usage of the option "type" is deprecated and will be removed in Symfony 3.0. As a value for the option you should provide the fully-qualified class name (FQCN) now as well.

• Passing type instances to Form::add(), FormBuilder::add() and the FormFactory::create*() methods is deprecated and will not be supported anymore in Symfony 3.0. Pass the fully-qualified class name of the type instead.

  Before:

  $form = $this->createForm(new MyType());

  After:

  $form = $this->createForm(MyType::class);

• Registering type extensions as a service with an alias which does not match the type returned by getExtendedType is now forbidden. Fix your implementation to define the right type.

• The alias option of the form.type_extension tag is deprecated in favor of the extended_type/extended-type option.

  Before:

  <service id="app.type_extension" class="Vendor\Form\Extension\MyTypeExtension">
      <tag name="form.type_extension" alias="text" />
  </service>

  After:

  <service id="app.type_extension" class="Vendor\Form\Extension\MyTypeExtension">
      <tag name="form.type_extension" extended-type="Symfony\Component\Form\Extension\Core\Type\TextType" />
  </service>

• The TimezoneType::getTimezones() method was deprecated and will be removed in Symfony 3.0. You should not use this method.

• The class ArrayKeyChoiceList was deprecated and will be removed in Symfony 3.0. Use ArrayChoiceList instead.

Let me know if I have forgot something important :)