Mystic Gohan was stronger than SSJ3 Goku during the Buu Arch. Reason being was because Goku just achieved SSJ3 and was not able to utilize the full power of it yet, while Gohan on the other hand only had to fight like normal and his hidden power would surface upon demand. Goku had to forcefully draw out the SSJ3 transformation where as later, it would be more natural.
Later, as Goku trains and gains more experience within his SSJ3 state and later unlocks SSJ4 (stupid transformation), he becomes strong enough that even in his SSJ3 mode, he can still defeat Mystic Gohan.
So at one point, yes, Mystic Gohan was stronger than SSJ3 Goku, but only for a short time. However, Mystic Gohan still exceeds the power of SSJ3 Gotenks.
Gohan but now when gokus fused with vegeta SSJ4
The question was solely SSJ3 Goku vs Mystic Gohan. That does not include any other transformations or fusions.