I know it won't be ideal, but are there pump action 12ga shotguns which could be used reasonably effectively for both trap and 3 gun competition? I need it to be pump for 3-gun...

 

Or do they require completely distinct qualities?

 

Melgamatic

I don't see why you can't use a Rem 870 or Moss 500. If you have to have a pump, these will do nicely. There are other pumps, but I have personally beaten the hell out of both types. They hold up well. You can get a 28" vent rib barrel for trap and whatever barrel you may need for 3 gun.

can you use a saiga 12 for 3gun?

BENELLI NOVA!!!! I have used mine for skeet, trap, sporting clays, pheasant/duck/goose hunting and I recently used it in my first 3 gun shoot. It is accurate, reliable and will shoot anything from old/unknown relooads to 3 1/2 inch Magnum slugs, no problems at all.