In this study, we investigated orbital period changes of three contact binaries DK Cyg, VW Boo and CV Cyg. From O-C analysis, it was found that DK Cyg indicates orbital period increase, whereas the periods of CV Cyg and VW Boo are decreasing. For DK Cyg, mass transfer from less massive component to more massive one was used to explain the variation in the period. On the other hand, the O-C changes of CV Cyg and VW Boo were clarified using the assumption of mass transfer from more massive component to less massive one. In addition, period variation in cyclic form are also seen for VW Boo and DK Cyg. Assuming the periodic variations of these systems caused by an additional component around the binary, we calculated the basic properties of the possible unseen component in the eclipsing pairs.