Goku Super Saiyan 3 was stronger than Mystic Gohan.

Fact 1) The official Japanese name for the episode where KB is defeated is “Goku is the strongest in the universe.”

Fact 2) Supreme Kai states “The absorptions (of the kais) weakened him.” KB > SB. The end.

Fact 3) Vegeta and Goku both stated that SS3 was more powerful than KB.

Fact 4) DBS Ep. 2, Vegeta states (in japanese) after defeating buu kakarot became the undisputed strongest in the universe.

Fact 5) Akira interview for Daizenshuu 4: “Goku, the always pure strongest in the universe.”

The end.

Lets make this old discussion rest.

No.