SS3 Goku is only available through Ultimate Battle 27.

Double Forward Flip: Press Half Circle Forward, X.

Energy Concentration: Hold Back, then press Forward, Circle.

Kamehameha: Press Half Circle Forward, Circle.

Chou Kamehameha: Press Down, Back, Forward, Circle.

Ultimate Kamehameha: Press Half Circle Back, Forward, Circle. Note: Ot takes up almost all of your Ki.