Standard general relativity treats gravitational waves as pure spacetime geometric fluctuations propagating in ideal vacuum, which cannot fully explain the observed waveform attenuation, high-frequency smoothing, and the origin of the nanohertz gravitational wave background. To resolve the inconsistency between theoretical prediction and actual cosmic observation, this paper proposes a three-component unified coupling model. We verify that real cosmic gravitational waves are comprehensively determined by three physical mechanisms: spacetime shock perturbation generated by compact object mergers, damping and scattering effects of the universal dark matter medium, and continuous spectral modulation caused by dark energy-driven cosmic expansion. We derive a self-consistent modified gravitational wave field equation and dispersion relation that naturally explains wave amplitude decay, frequency redshift, and large-scale low-frequency background formation. The model maintains logical compatibility with general relativity while introducing dark component correction terms, providing a new theoretical framework for studying gravitational wave propagation effects and dark cosmic physical properties.