The rear bumper brackets have to be different between 3100 and 3600/3800, as the former frame is 48 inches wide, and the latter 36 inches.
The front bumper brackets are likely different as the 3100/3600 frame horns are tapered toward the front while 3800 are parallel to each other.
I believe the bumpers have the same dimensions and bolt pattern (I have one of each front bumper, thin and thick, but they're buried currently so can't compare), but just the brackets are different.
That info is from looking at the FAM.
