In the model of photonic band gap, the entanglement dynamical evolution of a system where three two-level atoms interact with a heat reservoir is investigated. The entanglement among systems is described by using the pseudo-mode theory and the tripartite negative eigenvalue method. The effects of the heat-reservoir spectral density function, the detuning between the atoms and pseudo modes, and the interatomic dipole-dipole interaction strength on the three-atom entanglement evolution are analyzed by means of numerical calculation. The results show that in a perfect band gap model, the trapping phenomenon appears in the interatomic entanglement evolution versus time. The detuning between the atoms and pseudo modes makes the interatomic entanglement show a non-monotonic transition. The three-atom entanglement degree increases with the increase of the interatomic distance.