How different is Symfony3 from Symfony2?
Web Developers are used to big changes in their frameworks and tools. Being used to does not directly translated to being confortable. With the big change from Symfony in version from Symfony to Symfony2 fresh in memory - many are pondering how big of a jump it is to Symfony3.
The Symfony development team has listened to the feedback when their launched Symfony 3 late in 2015. Since that time the framework has made it's way to version 3.1, but the framework is still very similar to what Symfony2 developers are used to.
There have been some changes to directory structures and a number of deprecations, but largely developers confortable with Symfony2 will be so with 3.x iterations of the framework. This means that Symfony 3.0 was an evolution, not a revolution.
The low level architecture remains largely the same, but it does come with a number of improvements such as improved standards adoption with PSR-3, changes to console helpers being stateless and improved decoupling of the Symfony components from the framework.
Upgrading from a Symfony2 framework application to Symfony3 does not bring large immediate benefits and in fact Symfony 2.8 LTS will be supported until 2019. If there are features that you want from the Symfony 3.x branch then you'll need to upgrade as Symfony 2.x will no longer receive any new features.
For new applications, adopting the latest 3.x iteration is a no-brainer. It is worth remembering that a lot of the power behind the Symfony framework is attributable to the extensions to it. These are known as Bundles and the Symfony community highlighted the need for support early on and came up with instructions on how to upgrade bundles to Symfony 3.