I find that the 3-3-5 is all about athletic defenders, but you need bigger D linemen to occupy blockers.
I would recommend reading the following article if you want to learn a little bit more about the 3-3-5 and it might give you some ideas of how it should be run (I make no promises, because the in game 3-3-5 might not be programmed properly, but I find I can run the 4-3 Under similar to the real life version and it works fine):
http://www.grantland.com/story/_/id/...-3-3-5-defense
Also I would say mix up your blitzes, because you will usually be blitzing every play since you only have 3 D linemen. And since your D linemen are bigger I would avoid blitzes that drop them back into coverage because that is asking for them to get burned.