Here's the deal,Todd....
Blades is right, but it's only part of the story.
Grab the closest shell and measure it. A so called 3" shell is that long only when it's been fired. Until the crimp opens, it's more/less 2 3/4" long. IOW, a 3 inch shell fits nicely into a 2 3/4" chamber UNTIL IT'S FIRED! Then, the crimped portion expands into the forcing cone, pressure spikes way over SAAMI max, and it's not good for the longevity of either you or the shotgun.
FYI, shotguns and grenades have similar working pressures.
Also, most smiths capable of converting a 2 3/4" 870 to a 3" one are well aware of liability issues and wouldn't do it. 3" 870s are neither scarce nor expensive.
Frankenstein, my overpublicized parts 870, was converted to a 3" shotgun when I put it together. A gunsmith friend did the ejector change and other minor adjustments, and the barrel I used was a 3" from the getgo.
In truth,no great disadvantage ensues from having just a 2 3/4" 870, unless steel shot is mandated and not the alternatives for waterfowling. A standard 12 gauge WM is one very wonderful field gun, clays toy, and HD/CD tool. Enjoy...