To get Gotenks select either Kid Trunks or Goten, press the number 9 and youll see a fusion dance icon. Land on the ground then hold the right mouse button. YOull get 6 directions to press like in advanced melee. Only a lot faster.
If you do them correctly you get Gotenks if you do them wrong you get fat or skinny gotenks. Fusion lasts 5 minutes.
Same thing for Gogeta only that the characters are Orange GI Goku and Training Suit Vegeta.
Finally Vegetto you have to find 2 potarra earringson the map (and believe me thats hard cause those things are so small) or you wish for them from the dragon. Press number 9 twice and youll see a potara icon. Land and hold the right mouse button. You will fuse into Vegetto. This fusion lasts untill you die. Characters that have it are the same as for Gogeta.
That said fusion is just a different transformation of a character. Kinda like Metal Cooler.