Most 12ga rounds are 2 3/4”, not 3”.
My shotgun is designed for 3" shells. It will accept shorter shells, but I expect it's stated magazine capacity to be for the number of rounds of the longest shell it was designed for.
I've seen short "tactical" 12 gauge rounds designed so that you can get more of them into a tube mag. If they count that as a "round" and you can only have 5, then you probably can't have more than 3 3" rounds.