A　protocol　is　proposed　to　implement　a　three-qubit　phase　gate　for　photonic　qubits　in　a　three-mode　cavity.　The　idea　can　be　extended　to　directly　implement　a　N-qubit　phase　gate.　We　also　show　that　the　interaction　time　remains　unchanged　with　the　increasing　number　of　qubits.　In　addition,　the　influence　of　cavity　decay　and　atomic　spontaneous　emission　on　the　gate　fidelity　and　photon　loss　probability　is　also　discussed　by　numerical　calculation.