Modal synthesis methods have long been studied because the use of generalized coordinates makes it possible to reduce calculation costs. Our approach uses modes to describe each part of the assembly of several substructures, coupled with a fluid cavity. In a previous paper, we explained that omega2 developments could be used to minimize modal truncation. In the present paper, we consider a fluid-structure coupled system using a method called "triple modal synthesis." High order developments will be made to describe the fluid part. First, two kinds of formulation will be explained: in displacement and in force. Second, calculation using finite element methods will be processed.