San Ren Xiang Qi is a variant of Xiangqi with a special board for three players.
This game is mentioned in H.J.R. Murray's book "A History of Chess" on pages 133-134. He writes:
There are also enlarged games of chess in China. One of these is the San-kwo-k'i, or Game of the Three Kingdoms, which is described by v. Möllen-dorf. It is supposed to illustrate the war of the Three Kindoms, wei (blue), Shu (red), and Wu (green), A.D. 221-64. I give a diagram of the board; it will be noticed that the lines are not straight throughout, and that each kingdom faces the other two. The pieces consist of the usual 16 with, in addition, 2 new pieces [F] in each of the three armies. These are called: Red, Chuo (fire); Blue , Ch'i (banner); Green, Feng (wind). Their move is an extended Kt's leap, viz. two steps vertically or horizontally and then one diagonally. The game is said to be very complicated and difficult, but is not considered as interesting as the ordinary chess. When one of the Generals, who are named Wei, Shu, and Wu after the names of the three kingdoms, is mated, the player who has mated him removes the King from the board and adds the remainder of his army to his own.
The diagram he mentions can be found in the images section of this games entry page.