I went on a similar search a few years ago, and eventually found a Gallyon-branded boxlock made by AA Brown. 3" chambers, heavy proofs and 30" barrels choked F/F. It weighs around 7.5 lbs. There are some around but not a lot. Pigeon guns could be a reasonable alternative though not many have 3" chambers. If you are willing to pay to import something from the UK, they are more prevalent over there.

Such a long, long time to be gone, and a short time to be there.