A metallic-nanoshelled rectangular dielectric rod is proposed to flexibly enhance and tune the structural absorption. Due to the ultra-small thickness of the metallic-nanoshells, electromagnetic (EM) waves can penetrate into the rods and rectangular cavity resonances can be formed. At the cavity resonances, the strong EM wave-matter interaction results in an enhancement in the structural absorption by more than one order of magnitude. By stacking the nanoshelled rods, a three-dimensional (3D) woodpile photonic crystal with both the rectangular cavity resonance and the photonic band gap effect is realized. As a result, the structural absorption of the nanoshelled 3D photonic crystal is significantly enhanced to ~99.99% at the resonant wavelength.