Fight takes place on Supreme Kai's planet.
Goku is dead, so he can hold SSJ3 for longer than usual.
Time is right after Buu
Sorry if this has been done before. i'm new to this site and know near nothing :(
Mystic Gohan oneshots him. Goku SSJ3 was terrified of fighting Super Buu and wanted to fuse with Vegeta in order to ensure a victory. Gotenks SSJ3 fought evenly with Super Buu, therefor Gotenks SSJ3>Goku SSJ3. Mystic Gohan beat the ever living crap of Super Buu, therefore Mystic Gohan>Gotenks SSJ3>Goku SSJ3. Mystic Gohan takes this easily.