  In tripartite discrete systems, two classes of genuine tripartite
entanglement have been discovered, namely, the Greenberger-Horne-Zeilinger
(GHZ) class and the W class. To date, much research effort has been
concentrated on the polarization entangled three-photon GHZ and W states. Most
studies of continuous variable multiparticle entanglement have been focused on
Gaussian states. In this Brief Report, we examine two classes of three-photon
entangled states in space and time. One class is a three-mode three-photon
entangled state and the other is a two-mode triphoton state. These states show
behavior similar to the GHZ and W states when one of the photons is not
detected. The three-mode entangled state resembles a W state, while a two-mode
three-photon state resembles a GHZ state when one of the photons is traced
away. We characterize the distinction between these two states by comparing the
second-order correlation functions $G^{(2)}$ with the third-order correlation
function $G^{(3)}$.
