In many applications, heterogeneous structures are separated by some distance in a homogeneous medium. This homogenous space must be entirely meshed when volumic methods are used. This letter presents a hybrid formulation between the frequency-domain transmission line matrix (FDTLM) method and a field integral equation formulation that avoids the meshing of the homogeneous background in which complex objects are located. In addition, it prevents the use of absorbing boundary conditions. To simplify the theoretical development for the 3-D case, the problem of a rectangular waveguide with inductive objects is presented. Although invariance along the waveguide height with the fundamental mode excitation, the problem is treated by 3-D FDTLM nodes for validation. It is found that some computer cost reduction can be achieved as compared with the full meshing approach.