I got a 91 full size bronco Im puting a 3inch body lift does any body know how big tires I can run on it.  

It depends on the width of tire and rim.  I have a friend with a Bronco (I believe the same year) with 33/9.50R15 BFG A/T's on the factory 7" wide rims, totally stock suspension and they clear, but very minor rubbing some times.  33/12.50's on 10" rims might fit.  33/10.50's on 8" rims will clear fine.  Even 35/10.50's (yes 10.50's, some manufacture offer that size) on 8" rims might clear.