A low-loss and low-crosstalk silicon triplexer is proposed and realized for wavelength-division-multiplexed optical systems with the wavelength channels of 1310, 1490, and 1550 nm. The proposed on-chip triplexer is composed of three cascaded multimode-waveguide-grating (MWG)-based filters, which consist of an MWG and a two-mode (de)multiplexer. For all three wavelength channels, flat-top spectral responses are achieved experimentally, and their 3 bandwidths are 89, 19, and 7 nm, respectively. The excess losses are less than 1 dB. The crosstalks are -36--27 and -32--28 dB for channels 1490 and 1550 nm, respectively.