RE: 3" or 3.5" Remington 870 ???
You really have to have a need for 3.5 inch shells to buy a gun that will shoot them. Maybe geese that are flying really high. A 3.5 shell willpound you good. Its not worth it to me, but I never hunt geese.
I use light loads 2.75 in, 7 1/2 shot for turkeys. After shooting a 3.5 inchmag I just never found a use for them. Much worse than my 300 WinMag.
I suggest you shoot someone's 12 ga, 3.5 in Mag and see how it goes.