big three dragons; winning hand with pungs/kongs of all three dragon tiles
A high-scoring mahjong hand consisting of a pung or kong of each of the three dragon tiles (white, green, red).
If you win with a big three dragons hand, it's a yakuman.
When you collect pungs of the white, green, and red dragons, you complete a big three dragons hand.
From 大 (big) + 三元 (three fundamentals/origins), referring to the three dragon tiles in mahjong.