A triple-mode narrow-band bandpass filter is presented by exploiting three fundamental resonant modes, TE101, TE011 and TM110 modes, in a single air-filled rectangular cavity. Compared with the conventional multimode cavity filter, the proposed filter is to simultaneously excite these three resonant modes without installing any cuts or screws inside the cavity, so as to keep the integrity of the cavity and decrease the fabrication cost. In particular, an inclined U-shaped slot is utilised at each of the microstrip feed line to properly excite these three modes such that not only three poles are achieved within the desired passband but also a pair of transmission zeros are generated in the lower and upper stop-bands. A single metal rectangular filter is finally fabricated and tested. The measured results have well validated the simulated results.