Si Ma San, also called Four Horses, is a 3-point unit on the anteriolateral thigh on (or near) the Stomach Channel. This is a great point combination for skin stuff - hives, eczema and more. The main point is the Si Ma Zhong point with two dao ma points 2 cun proximal and distal to the main point.
To find these three points, start with Si Ma Zhong.
Si Ma Zhong (Four Horses Center - 88.17) is level with Gallbladder 31, but is 3 cun anterior to the point and on the Stomach channel. Sometimes you'll see this written as "four fingers anterior to Gb 31.
Si Ma Shang (Four Horses Upper - 88.18), the 1st of the two dao ma points ,is located 2 cun proximal to Si Ma Zhong.
Si Ma Xia (Four Horses Lower - 88.19), the 2nd of the two dao ma points, is located 2 cun distal to Si Ma Zhong.
If you are treating disease of the skin, you can needle more shallowly. If you are treating deeper diseases, you needle more deeply. Susan Johnson has an excellent write-up on this that explains this theory more thoroughly.