Metamaterial absorbers can confine and completely absorb incident electromagnetic waves to the subwavelength scale and have promising applications in detection, thermal emitters, energy harvesting, cooling, etc. The multiband metamaterial absorbers reported thus far are mainly the perfect absorption of multiple similar wavelengths in a specific wavelength range. Achieving multiwavelength absorption over a wide spectral range requires the combined work of multiple structures. Based on the three-layer structure of the titanium cross resonator-silicon nitride dielectric layer-titanium reflective layer, a triple-band metamaterial absorber with operating wavelengths spanning midwave infrared, longwave infrared, and very longwave infrared was designed and numerically simulated. Using the propagating surface plasmon resonance, the localized surface plasmon resonance, and the silicon nitride intrinsic absorption mode excited by the metamaterial absorber, high absorption reached 97.3%, 94.4%, and 93.6% at wavelengths of 4.8 μm, 9.1 μm and 18 μm respectively. Meanwhile, the wavelengths of the three absorption peaks can be flexibly manipulated by changing the geometric parameters of the metamaterial absorber, and the absorber exhibits insensitivity to polarization and incident angle. The materials used in this work are commonly used in existing processes and have application prospects in gas detection and infrared imaging.