A pung is three identical tiles. In American play you may call a discard to complete it, then expose the group.
Chinese tables also pung. The difference is elsewhere: American mahjong has no chow, and a win is a printed line.
A joker may finish a pung. It may not finish a pair.
Call it, then expose it
In American play you may pung a discard. The three tiles then sit face up on the rack. If the card wanted that group concealed, the line is broken.
A joker can finish a pung. It cannot finish a pair. There is no chow, so you call only for identical tiles or to declare a win.