This paper is the third followed by the two companion papers dealing with ultimate longitudinal strength analysis of container ships considering effects of bottom local loads. The major causes of reduction of ultimate hull girder strength due to local loads were discussed based on the finite element analysis of a hold model in the Part 1. The extended Smith's method was developed in Part 2, which is a practical method of progressive collapse analysis of a hull girder under combined longitudinal bending moment and bottom local loads. The objective of this paper is to develop a simplified estimation method of ultimate longitudinal bending strength of a hull girder which can consider the effect of bottom local loads. The proposed method uses an ultimate strength interaction formula on basis of first failure of hull girder. The first failure in compression may be occurred at outer bottom plate near a partial bulk head or inner bottom plate near a water tight bulkhead depending on the deformation of double bottom. The effect of first failure location on hull girder collapse behavior is investigated by the finite element analysis, and it is found that the outer bottom collapse is a direct cause to the hull girder collapse even if the collapse precedes at inner bottom plate. Therefore, the proposed method targets the case of first compressive failure at outer bottom plate. The ultimate longitudinal strength estimated by proposed method is compared with that by finite element method in order to validate its estimation accuracy.