We experimentally demonstrate a novel, to the best of our knowledge, three-port grating coupler (TPGC) for dual-wavelength-band operation. The TPGC can couple light to/from two ports for S/C-band with polarization diversity and a third port for O-band. Such a coupling scheme could be applied in an integrated wavelength-division-multiplexing passive optical network (PON) unit, benefiting a polarization-diverse receiver for downstream S/C-band and transmitter for upstream O-band. The device is fabricated on a multi-project wafer, with peak coupling efficiency measured to be -3.8dB and -5.4dB for O-band and S/C-band, respectively. We also present a proof-of-concept demonstration for 10-Gb/s PON transmission based on the TPGC.