I've had good luck with a factory super full choke and 3" Winchester H.V. #4 in a 3" 870. The 870's in general seem to like a .665 choke.
I've also had good luck with a Primos Jellyhead .665 and Federal Flite Controls in 3.5", #5.
The NWTF and Old Gobbler web sites have really good fourms discussing guns and shooting. You can read all day about various choke/shell combos. Alot of those guys REALLY get into the shooting aspect of turkey hunting. One of the guys that post alot on both fourms is allaboutshooting he is a tester for alot of companies the make and sell chokes and shells.
If you'll watch quite often there are choke tubes that are sold on the classified sections of fourms, including this one, that are really good buys.
Good luck!!