They really don't. As we know, both Super Buu and Mystic Gohan are stronger than Goku SSJ3, in terms of strength, speed and raw power, but I doubt the difference would be that great. Also, Goku's whole 'determination' factor would let him hold his own against Super Buu (the weaker of the two) for a while, at least (we haven't even seen SSJ3 Goku's full power, though I accept that he still isn't capable of killing Super Buu). But lets consider Janemba, who was shown to be stronger than Goku SSJ3. Gogeta SSJ took a punch from Janemba as though it was nothing. Considering the strength of base Vegito, and that Gogeta was SSJ, he should be able to solo team 2 without much difficulty.