A triple-band bandpass filter is presented by exploiting three fundamental resonant modes, TE ₁₀₁ , TE ₀₁₁ and TM ₁₁₀ mode, in a single air-filled rectangular cavity. An inclined slot with U-shape is utilized at each of microstrip feed line to properly excite these three modes and generate one transmission zero (TZ) between any of two adjacent modes. Hence, each mode could contribute to one passband alone. Compared to the conventional multi-mode cavity filter, the proposed filter is to simultaneously excite these three resonant modes without installing any cuts or screw inside the cavity, so as to keep the integrity of the cavity and decrease the fabrication cost. In theory, a bandpass filter is 20-dB in-band return loss and 30-dB between-passbands attenuation is obtained. A single metal rectangular filter is in final fabricated and tested. The measured results have well validated with the predicted results.