Plays essential roles both in the penetration of the viral genome into the bacterial host via pilus retraction and in the extrusion process. During the initial step of infection, G3P mediates adsorption of the phage to its primary receptor, the tip of host type IV PAK pilus. Subsequent interaction with the host entry receptor induces penetration of the viral DNA into the host cytoplasm. In the extrusion process, G3P mediates the release of the membrane-anchored virion from the cell via its C-terminal domain. Interacts with G6P; this interaction is required for proper integration of G3P and G6P into the virion. Interacts with G8P (By similarity). Prior to assembly, G3P is found associated with the bacterial host inner membrane. There are about five copies of this protein per mature phage that are located on the head side of the filamentous virion. Belongs to the inovirus G3P protein family.