From the following points of view, periodic response of three dimensional floating framed structures is studied in this paper. What is worked on is three dimensional floating framed structures which consist of plural columns with immersed body and connecting members. (1) The relationship between member force response and motion response should be clarified for the overall structural design. (2) The amplitude and phase lag of member forces at member junctions are needed for the local structural design of junctions. (3) The effects of flexibility of connecting members on the motion will be important for large scale floating framed structures in future. A new analytical method based on the fundamental concepts studied by authors so far is developed to discuss the problems mentioned above. On the other hand, model tests of several types of three dimensional floating structures were conducted to evaluate the applicability and the accuracy of the new method. The calculations are compared with the experiments and several comments on estimation of member forces of connecting members are given from the calculated results.