|Three concealed triplets
Sanankou 「三暗刻」 is a yaku, consisting of three concealed triplets (ankou). The third must not be completed off of another player's discard, as this would not count as concealed. However, if it is completed with a tsumo, then the yaku is awarded. A concealed kan is also counted as a concealed triplet. The hand this yaku is awarded to may be open or closed, as long as it meets the conditions. This yaku is awarded 2 han.
Tile pattern cases
1. Case where the yaku is guaranteed:
2. Case where the hand must be won by tsumo:
3. Case where the hand is won by discard.
Note: A tsumo results in the suuankou yakuman.
A natural combination involves toitoi. With one tile call, the hand needs to win by tsumo; otherwise by ron, only toitoi is counted. This premise is a similar case involving suuankou, which counts as yakuman via tsumo. Otherwise, it may not.
A tenpai hand with sanankou with toitoi is tenpai for suuankou. When this hand wins by discard, then the hand is not counted as suuankou. Instead, it is the combination of sanankou and toitoi. The fourth triplet completed by the discarded tile counts as an "open triplet", rather than a fourth closed one.
- Sanankou in Japanese Wikipedia