Sanankou
San ankou (三暗刻) is a yaku, consisting of three concealed triplets (ankou). In the case of having three triplets, the third one could not be completed with another player's discard, as this would not count as being concealed. However, if it is completed with a tsumo, then the yaku is awarded. Ankan is also counted as a closed triplet. The hand this yaku is awarded to may be open or closed, as long as it meets the coniditions. This yaku is awarded 2 han.
Tile pattern
For a guaranteed rewarded yaku:
Winning tile:
In the case where the wait must be won by tsumo:
Winning tiles (must be self-drawn):
English reference
In English, this yaku is commonly referred as "Three concealed triplets".