Lift the sofa up and carry it over to the 31-inch doorway. Turn it slightly on its side to reduce its width. Continue turning it on its side, or on end, until you get an angle that will fit through the doorway.
Take apart the sofa yourself if you can't get an angle for it to fit through the doorway. To take apart the sofa and get it through the doorway, you'll need to turn it over. If you're outside, lay a plastic sheet down to protect the fabric. Then, take the fabric off the underside. It should peel back to reveal the sofa arms on either side. Pull these out to reduce the size of the sofa. Carry it through the doorway while keeping it upturned, and reassemble once you get inside.