FOSUserBundle and Symfony 3.0?

This command worked for me with Symfony 3.2.1:

composer require friendsofsymfony/user-bundle "~2.0@dev"

At the moment, there isn't a tagged version (official version) for support of the new framework version, BTW, in this github issue says that:

in the the master branch is now compatible with Symfony.

So try with this composer configuration

friendsofsymfony/user-bundle: "dev-master"

Hope this help