During Tag Team matches, hold L2 and press Left + R3. Note: This requires three Favorite gauge bars and the correct characters to fuse with; such as Goten and Trunks to fuse into Gotenks.

Fusion list

Goten and Trunks: Gotenks (Fusion)
Goku and Vegeta: Gogeta (Fusion)
Goku and Vegeta: Vegito (Potara)