I don't know what models or mfg's you are looking at ....but 3" chambers on most any "field" gun / including pump guns ...have been very common for over 30 years ....
As a sign of the times, we were in a Big-5 last week looking for some 2 3/4-inch 12 ga turkey loads. But, all their turkey loads were 3-inch, and their only 2 3/4" loads were dove/target loads in #7-1/2 shot and smaller. The young man behind the counter was amazed that we had a field gun that wasn't chambered for 3-inch shells.
2 3/4" and 3" are listed to the same maximum pressure while 3.5" is higher.
conveting guns to 3" is relatively easy. converting to 3.5" requires heavier barrels.
As I explained to Brent (in response to his PM on aging ), we were trying to get some 2-3/4" shells for an old Savage 775A that my neighbor got from his son's father-in-law. I'm not exactly sure, but it think it was made soon after WW II.
To answer kx592's question: Why not get a 3 and use 2 3/4 if thats what you want? Whats the issue General purpose, or field guns are typically 3-inch. Target guns shoot 2 3/4-inch shells and are chambered accordingly.
You can plink with .38 specials in your .357 rem mag revolver; but, for bullseye work you'll want something chambered for .38 special mid-range ammo. The same applies to shotguns. You can have fun in the shotgun sports shooting 2 3/4-inch shells in a 3-inch chamber; but, if you step up to a real target gun, you'll want 2 3/4-inch.