Merkels are not lightweights in the sxs world. The pressure standard for a 3" 12ga is the same as for 2 3/4", so in that respect, it should not be an issue. But I'd agree with Joe (for once!) that I would not bother--unless maybe you intend to shoot steel in the gun, in which case 3" does give you some advantages. If that's the case, you'd need to consider opening the chokes if they're tighter than mod.