He loves older kids who can converse with him and do fully cooperative group complex pretend play (which he adores!) so the social-fit is good (I just hope it's enough!).
There will be no great fix to this situation as there are *lots* of differences between 6 and 7 yo children and 3 yo children, between 12 yo and 8 yo, etc., and being capable of the intellectual work of older children will not erase the other differences. The social fit has to be both ways, so that it's not only whether your son likes the fit but whether the other children do too. At 6 or 7, my children would not have chosen to play with a 3 yo when they had lots of age peers available.
I would look for opportunities to find age peers for your son as well as good educational fits. These do not have to be the same activities. If your son needs social outlets, I'd find sports, clubs, or other age and interest based activities that he could join. For radically accelerated kids, having some interaction with others in the same age range seems to be really important.
I agree with the other posters that 3 is too soon to plan long-term education decisions. Most early schooling is play based and as long as your son likes to play, he may enjoy early schooling even if it isn't appropriate academically.
