We present a robust (n, n )-threshold scheme for multiparty quantum secret sharing of key over two collectivenoise channels (i.e., the collective dephasing channel and the collective rotating channel) via three-photon mixed states.In our scheme, only if all the sharers collaborate together can they establish a joint key with the message sender and extract the secret message from the sender's encrypted message. This scheme can be implemented using only a Bell singlet, a one-qubit state and polarization identification of single photon, so it is completely feasible according to the present-day technique.