How can you get Goku ssj5 on BT3

You can't, because the SSJ5 level hasn't been officially created by Akira Toriyama (the creator of Dragon Ball).
But if you really want to play against a SSJ5 Goku, you can search for a hack of Budokai 3 (NOT Budokai Tenkaichi 3, notice that) named Dragon Ball AF.