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I just made my first Chuurenpoutou ever, but this one was in 3er though at the SEGA MJ site.

Started with two and two - wound up having to wait for the last for it, but my lesser valuable waits were and - got it on a ron, from the same guy who threw out a earlier in the hand.

That said, is Chuurenpoutou that tough even in 3er? - it is definitely up there in difficulty in regular 4er for sure, much less the Junsei version of it...

Seen a lot of 3-player Yakuman wins, but don't recall seeing any collated stats.

Even though the 2-8 Manzu tiles are removed in 3P and you can't make a flush hand with that suit, I think you're still over four times more likely to draw any 13+ random tiles of the same suit (in your starting hand and during play) in 3P compared to 4P, but with those better odds you're also more likely to make Tenpai - and to get Tenpai earlier - and then you have more opportunities to get a winning tile or to develop non-Chuuren into Chuuren and then get the win. Declaring any north tiles as Nuki Dora in 3P would give extra draws too. With 28 of 136 tiles removed from play, you also have a better chance of a win off each draw/discard once you're Tenpai.

You'd need a sophisticated analysis to give the actual probability or a very large sample of 3P match stats.

(Just for reference/fun, in Tenhou's 4P archive Chuuren occurs once per 123k hand wins. There have been twenty Chuuren wins so far this month out of around three thousand Yakuman gets. Pure Chuuren occurs once per 1768k wins - making it the second rarest after Suu Kantsu - but there have been two this month already.)

Sega MJ doesn't have the Declare North as Dora option for 3er - a set of North Wind tiles counts as yakuhai, as if it were a dragon set.