Most newer 20 gauges are chambered in 3" mag regardless of make. Personally, I see little advantage in using 3" magnums for most hunting, one exception is waterfowl hunting with steel shot, another is turkey hunting.
I've owned a few 870's over the years, mostly the Wingmaster version. Right now I have a 20 gauge Express model I bought used about 7 or 8 years ago. Never had any problems with any of them.