In this brief, a low-profile triple-fed high-isolation triplex filtering antenna is proposed based on the substrate integrated waveguide (SIW), for the first time. By co-sharing radiators, constructing common feeding resonant cavities and loading additional metal via holes, a closely-spaced-channel triplexer topology is developed with three adjustable frequency passbands. To verify the proposed concept, a prototype operating at 6.68 GHz, 7.52 GHz and 8.0 GHz is designed, simulated and measured. It exhibits high channel-to-channel isolation of over 21.83 dB as well as good passband gains of 7.1 dBi, 4.55 dBi and 5.99 dBi, respectively.