If you want to upgrade your Symfony project to Symfony 3, you must check many different points. There is a page in the Symfony documentation that explains how to upgrade a major version.
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You can find a detailed article on upgrade symfony from 2.8 to any 3.x version here: https://www.cloudways.com/blog/upgrade-symfony-2-to-3/
You just need to check if there are any deprecated functions or not in your application before upgrading.
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