Vegitto super saiyan 3 is from a comic book series called multiverse where in different universes there could have had different outcomes to fights. In this case Goku and Vegeta stayed fused into Vegitto and defeated kid buu. Later in the universal tournament Vegitto from universe 16 fights Broly of universe 20. Ultimately Broly has gained so much strength that Vegitto has to resort to the super saiyan 3 transformation and land a final kamehameha and defeat broly once and for all!