Yeah, I think the 3+Plug is just for birds.
Deer is 8 shells/cartridges max no plug needed.
No, the limit is for small game also. Only deer and bear is it 8. I’d have to go back and poke through the regs to see if the limit applies to pest species like nutria and ground hogs.
http://www.eregulations.com/maryland/hunting/small-game-hunting/