The body is sometimes acted by two or three force members and we need to find the radius of gyration for the same. The difference between the two and the three force members is:

The former is acting on two points in the body while the latter is on three points

The definition of the two force member only defines that the forces are being acted on the two points on the body. So does is the definition of the three forces members. The points of action of the three forces are three.