I know this is a little old, but having just come back (last night) from hanging out with a kid EXACTLY DS's age (they're like 10 days apart or something like that), I wanted to chime in again.
After another adult and I took turns letting the three year-olds catch and tackle us, we could sort of casually bow out and go back to sipping beers around the bonfire, and DS sort of picked up the leadership mantle and started directing... "Now you chase me!" "Now I'll chase you!"
When we had been inside earlier and they were trying to play with cars together (something DS does EXTENSIVELY and elaborately on a regular basis in his own little world), we ran into problems because he just had a certain way it wanted to be done. He is usually the king of his little vehicle-play world, and the other little girl stared blankly when he started going on about the toy train being a steam engine and needing coal. When they tried to race them, well, let's just say all hell broke loose. He is very particular about how he plays with his cars, and is used to doing it alone and at-will, or with an adult.
I guess helping find something that is a "different" kind of play than what he usually does if left to his own devices helped. Or something where precocity doesn't make much difference. If left to his own devices, DS would have carried my niece's toy cars around all night (the joy of being at a different house, to DS, is that WOW MOM DID YOU SEE THERE ARE DIFFERENT TOY VEHICLES HERE?!?!?!?!) and played with them non-stop. With a little direction, he made a new 'friend.'
