  We consider polarization states of three photons, each in the same given
spectral-angular mode. A general form of such states is a superposition of four
basic three-photon polarization modes, to be referred to as three-photon
polarization ququarts. All such states can be considered as consisting of one-
and two-photon parts, which can be entangled with each other. The degrees of
entanglement and polarization as well as the Schmidt decomposition and Stokes
vectors of three-photon polarization ququarts are found and discussed.
