Ultimate Gohan is actually likely way stronger than SSJ3 (at least either character that attained SSJ3). AT stated that if either SSJ3 character was stronger than the other, it was Gotenks, who only gained the upper hand against Super Buu near the end of the fight until his power began to wane and he defused. This is the same Buu that Gohan thrashed for an episode and a half, as well as the one who Goku stated he and Vegeta were no match for without fusion.
Ultimate Gohan is generally regarded as the strongest character that wasn't fusion or absorption. Counting both, the strongest character in the series canon is Vegito.
well in base form (no mystic or ssj) goku hands down but lets say goku wuz mystic then goku still wins and gohan can barely reach giji sjj2 while goku's a ssj4 he wins again lets also not forget goku knows the kaioken but gohan is ties for 2nd with vegeta cuz vegeta knows attacks like big bang or final flash