I bought a plain-jane Remington 870 Express, last year, with the extended magazine and 18-1/2" barrel, at Academy Sports for $319.
Mine was a 20 gauge, which I specifically wanted, for my wife's benefit. I have since added a 20" rifled slug barrel, to expand it's function and allow me to shoot sabot slugs. Sabots through a rifled barrel are quite accurate out to a hundred yards or more, making the 20 gauge an outstanding .58 caliber hog rifle. I carry it in a home-made sling in my Kawasaki Mule, and I now don't feel like I have a gun that I never get to use, as was the case before. There aren't many uses for an 18-1/2" shotgun, other than self defense or snake killing.
The Mossberg and the 870 Express are of approximately the same quality, and most LEOs and military guys don't make much distinction between them. They are both very good. I like 870s best, having owned both, but I can't really say why.
The Nitro and the Tactical models that you mention are basically the same gun as what I own, but with a few add-ons so they can appeal to folks who require the adjective 'tactical' to be included in any description of a SD type weapon. The picatinny rail would be nice, in case you wanted to add a scope for firing slugs, but other than that, I don't see much that would merit another $100 plus in the price.
In answer to your question about the 3" chamber - yes, that means it will shoot shells 3" long or less. 2-3/4" shells are fine, actually preferable, in a 12 gauge SD gun, where reduced recoil is a bigger plus than even greater destructive capability.