I might be wrong, as I haven’t tested it, but if you just have your RouteCollection creating PHP file saved under /app/config/routing.php and no other routing file in the /app/config directory, Symfony should just pick it up and use it.
Well, no. During instalation, symfony3 has created all its app/config files (including routing) in .yml and whatever I tries, will always relies on routing.yml for routing.