If the 870 fits you for field, it would work well.
There are so many parts & options for 870s that none of these are issues.
The only one is Ive never seen an extension for 20 gauges, but they probably exist somewhere.
Im sure the 20 gauge youth comes in 3 inch mag because my duck hunting buddy's daughter uses one for ducks. Google it...
Good luck
Alternative: Browning BPS upland in 20. Or micro midas. Not as many aftermarket options. I have a bps upland in the bedroom closet when my short barreled 12 is changed over for turkey hunting mode.
Last edited by humdinger; 10/20/16.
