@SpeedForceSpider: Goku SSJ3 and Gotenks SSJ3 imo are pretty even let me explain: Goku SSJ3 has never EVER shown his true power in the main DBZ episodes, not agains Fat Buu witch was painfully clear, and not even agains Kid Buu, he was letting the fight drag on to let Vegeta have a shot at him, the only time we could say he was going all-out was againts Janemba and against Hirudegran, when he fought agains Janemba he was dead and was in Outer World so he would not suffer from stamina draining with was confirmed multiple times in the show. IMO Kid Buu and Super Buu are close in power i'm still not sure who is definitly stronger but i think its safe to say Super Buu and Kid Buu aren't that far away in power. Gotenks SSJ3 himself, rather Goten confirmed that Super Buu and Gotenks where about even and it definitly shows in there battle. Goku however is a better fighter then Gotenks so he has that small extra advantage.
Even when Gotenks came out of the HBTC, Superme Kai commented on Gotenks SSJ3 transformation and asked Goku about it:
Supreme Kai: Did you say SSJ3
Goku: yeah, you didn't know we saiyans could go beyond a SSJ? let me try to explain, we all have the abilitie to go beyond an SSJ, but this! this is a lvl far above that.
Supreme Kai: So, does that mean Gotenks is stronger than you?
Goku: well...i could do it to, but man these boys..they reached a lvl i could not reach until my life was over
Here Goku doesn't say Gotenks is stronger than him he was only impressed that they became SSJ3 faster than him. Like i said we never saw Goku SSJ3 go all-out, in everyfight he was in he was always holding back.
I previously said that Mystic Gohan would win but i didn't read closely as it says this is Goku at the very end of Z, Goku ofcourse always trains mabye not that intensive as before but still. I would say this is a close battle neither of them is stomping eachother. Correct me if i'm wrong on something :)