Many efforts have been devoted to the design of photonic microcavities and the utilization of them in optical sensing, imaging, optomechanics, lasing, and micromanipulation. Cavities supporting spectrally close multiple resonance modes can favor energy exchange among the modes and the ambient, enabling nontrivial coherent dynamics that are useful in wave manipulation. Coupling between multimode cavities with multiple waveguides is a significant theme for optical, terahertz, and microwave signal control, but remains largely unexplored. Here we present a phenomenological modeling based on the coupled mode theory (CMT) that fully accounts for the interplays between such a cavity and surrounding waveguide structures in a generic situation with asymmetric coupling rates between the guiding channels and the resonant modes. It is shown that the waveguide-cavity couplings are crucial for the energy steering between the modes and their far-field radiation and provide flexible control over waveguide transmission featured with a multiple Fano line profile. Numerical simulations were conducted for a spoof plasmonic cavity waveguides system working at sub-GHz band to demonstrate these effects. The results are in good agreement with the CMT prediction.