Ittsuu vs sanshoku
Ittsuu and sanshoku doujun are similar but incompatible yaku, because hand does not have enough space for both. When a hand has the potential to score one or the other, a player must choose which to aim for.
Formation
Ittsuu and sanshoku require 3 specific sequences. They are both worth 2 han closed, and 1 han open.
However, according to monthly data in Tenhou.net, sanshoku is around twice as frequent as ittsu. This is due to a few reasons:
- It is easier to make a 345 sequence than a 123 sequence. To make 345, a player can have {34, 35, 45} in hand. To make 123, a player can only have {12, 23} in hand.
- Players prefer middle tiles over terminals for tile efficiency reasons. This means they are more likely to have a 34 or 45 ryanmen than a 12 penchan or 23 ryanmen. Thus, it is easier to "incedentially" form sanshoku than it is
Sanshoku is compatible with tanyao and chanta/junchan, while ittsu is compatible with honitsu/chinitsu.
Decision
Typically, it is best to go for the yaku that is easier to confirm / takes less steps to confirm. If a hand is one tile away from ittsuu, but two tiles from sanshoku, then it should obviously go for ittsu all else equal.
Other than that, consider what other yaku the hand can achieve. A hand that can score pinfu, and can choose between a 12 penchan (for ittsu) or 45 ryanmen (For sanshoku), should pick sanshoku to guarantee pinfu.