The hand
Big three dragons needs pungs or kongs of all three dragons, plus any set and a pair. It is a limit hand in Hong Kong, a yakuman in riichi and an 88-point hand in MCR.
Little three dragons
Little three dragons has two dragon pungs and a pair of the third. It is valuable but not a limit hand.
The responsibility rule
Many rule sets make a player who discards the dragon that lets someone complete big three dragons (after two dragon pungs are already exposed) responsible for paying. Be careful with dragons late in the hand.
Going for it
With two dragon pairs or a pung and a pair early, the hand is worth considering. Without them, it is a long shot.
