Should be able to have a gunsmith lengthen the forcing cone so you can shoot the 3".  My 1100 is the non-magnum version and my smith did some additional work so it will safely handle the 3" mag loads.  He also ported my barrel.  I have shot this set up out to 50 on paper with the HTL shells.  I killed a spring bird at 44 yards 2 years ago.  Just remember that in the woods the branches will eat up shot and with a 20 ga you have less to work with.  That being said, the 20 is perfect for numerous situations.
CB