Is the mystic form stronger than SSJ3? We know as a fact that SSJ &lt; SSJ2 &lt; SSJ3 &lt; SSG &lt; SSB . But what about the mystic form that probably Gohan is going to recover? Is there any official info / evidence that shows how strong it is compared to the other forms?
My take on this is that his "mystic" form is just him unleashing all his power without turning into a SSJ which puts some strain on his body / wastes some energy. 

Basically even if he could go SSJ3 there is no need for that because he can bring all of that energy out in this "mystic" form without any side effects. So no matter how strong he becomes with this he does not need to transform (regarding the normal Saiyan transformation).

Now regarding the "god" transformation (if he ever reaches it), I'm not sure. Since we have a new kind of ki involved maybe he can not bring that out without transforming into a SSG but that is even more based on speculation.

In the end, Mystic form > all other normal SSJ forms because it enables him to draw out all energy without any side effects.