We have to study slowly varying wave drift forces on very large floating structures (VLFS) carefully when we design a mooring system, because it is possible that the frequency of the slowly varying wave drift forces will be almost the same as one of the natural frequencies of VLFS-mooring systems. The aim of our research is to develop an estimation method of such forces on VLFS in the seas. In the 1st and the 2nd report, we presented a legitimate estimation method in which we can evaluate slowly varying wave drift forces easily by means of the integration of the squared relative wave height and we validated that method with model tests and with numerical calculations. In this 3rd report, we estimated slowly varying wave drift forces on so-called Mega-Float Phase II Model with applying the method, i.e. with using measured relative wave height, and also we attempted to evaluate the incident wave from relative wave height. That is, this report shows the possibility of the estimation of slowly varying wave drift forces on VLFS from the incident waves.