have a rem 870super magnum 3.5in , and a 20 in rem barrell that came off a 3in gun. is there a reamer to allow the 3in barrel to chamber the 3.5in shells, ,,,, khh

There is most likely a reamer for 12ga 3.5" shells

But I would first lay the barrels side by side and measure the OD's to be sure it is even possible. The barrel may become to thin near the lead in to the bore.