I don't remember the details, but at one time I've heard there was a way to make 3" STEEL shells work in a standard 1100. But there is more than the barrel involved. If you were to try to shoot 3" lead with that combo you'd have problems, this was 3" steel only. The 3" shells generate a lot more pressure, the ejector isn't set up for 3" shells nor is the gas system.
While it might be possible to make it happen I don't think it'd be worth the effort. You can do better with 2 3/4" shells than you think on duck. The 3" shells are more of an advantage on geese at longer ranges anyway.
I'd use what I had for a season. If you decide you really like waterfowl hunting and want to get involved in a big way you'll want to buy a new gun that would be a better waterfowl gun anyway.