Diffusion wake that accompanies the jet-induced Mach cone provides a unique probe of the properties of quark-gluon plasma in high-energy heavy-ion collisions. It can lead to a depletion of soft hadrons in the opposite direction of the propagating jet. We explore the 3D structure of the diffusion wake induced by $\gamma$ triggered jets in Pb+Pb collisions at the LHC energy within the coupled linear Boltzmann transport and hydro model. We find a valley structure caused by the diffusion wake on top of the initial multiple parton interaction (MPI) ridge in rapidity and azimuthal angle. This results in a double-peak structure in the rapidity distribution of soft hadrons in the opposite direction of the jets as an unambiguous signal of the diffusion wake. We use a two-Gaussian fitting method to extract the diffusion wake and MPI contributions to the double peak. The diffusion wake valley is found to deepen with the jet energy loss as characterized by the $\gamma$-jet asymmetry. Its sensitivity to medium properties such as the equation of state and shear viscosity is also studied.