The cross-spectral density matrix in three points′ interference pattern is derived, and the variation of polarization properties of partially coherent beams under this condition is given. A comparation between the cases of unrelated and related field components of x-direction and y-direction is studied, and compared with two points′ interference pattern in the same conditions. It is shown that the degree of polarization is oscillating in the observation plane. The farther away from the center, the more intense the oscillation is. The degree of polarization at different positions tends to the stable value after a long-propagation distance, which is related to the parameters of Bxy and δxy. The farther away from the center, the longer-propagation distance that the degree of polarization tends to be stable. The degree of polarization on the axes is closely related to the initial degree of coherence of three points. In particular, when field components of x-direction and y-direction are related, the degree of polarization at the corresponding point is much larger than the case of unrelated field components.