Welcome to THR!
You need a 3 1/2" 12 gauge... why? Waterfowling? If you have a genuine need for hoisting that much payload, you would be MUCH better served by a 10 gauge than a 12.
My immediate suspicion when I see someone express a "need" for a 3.5" 12 ga., FWIW, is that lots of folks fall victim to the good old American "bigger is better" mindset. Where 12 ga. shotguns are concerned, it just isn't so. Most of us get by just fine with 2 3/4" shells in 12 gauges. Some few actually need 3" loads for waterfowling or turkey hunting, etc. But there is little if any practical advantage to the 3.5" loads, or to the guns made to handle them. Most fishing lures are designed to catch fishermen not fish, the same thing goes for some shotguns.
Be that as it may, tis your money- spend it as you wish.
My advice is to pick whatever make/model of pumpgun that suits you best. Get a 26-30" vent ribbed barrel to fit that gun for clay games, small game hunting and wingshooting, and get an 18-20" smoothbore barrel with rifle type sights for defensive use and big game hunting. That should cover your basic needs, there are lots and lots of one-shotgun households all over the country who get by just fine with that sort of setup.
The most versatile and available of pumpguns is the Remington 870. Mossberg fans prattle about the "superiority" of the control layout on their favorites- in time your fingers will learn where everything is on their own, I assure you, and the alleged inferiority of the 870 in that regard will not matter one whit.
There are more than 8,000,000 (that's 8 million) 870s out there, along with a corresponding number of spare barrels, parts, accessories both needed and useless, etc. You can get an 870 and take advantage of this. Or, you can saddle yourself with something "better" which lacks this level of market support. Your call. Just learn the models available, what differences there are and aren't, and pick your gun and accessories accordingly.
Have fun and stay safe,
lpl/nc
