in my opinion , the 3 inch shell should actually pattern better from the 3,5 inch chamber as the longer chamber is not much , if any different, than a gun that has had the forcing cone lengthened, which will allow the shot column to enter the barrel with less stress and deformation of the shot. good round shot , without flat spots or egg shape , will fly straighter , resulting in a tighter pattern.
i shoot a fixed choke double. left/ full ,,right / modified. when the gun was new there was very little difference in pattern density or size at 25 yards. i made a tool to lengthen the cone of the full barrel ,which improved the pattern from that barrel , shooting 2 3/4 inch shells only.
CLARK ,,,please correct me if i'm wrong .