Technically it is L1, L2, L3, Common, fifth wire would be chassis ground. Unfortunately most electrical panels (in the US, maybe different elsewhere) connect the common to the ground pin of the building so unless the site was wired for 5 wire 3 phase or 4 wire 220VAC Single (L1, L2, Common, Ground) it is just a duplication of the wire.
Ideally there would be a second ground pin located near the electrical panel for local ground, and the common would be connected to the pin closest to the mains. This will ensure if there is a surge that it limits the damage to the local area and not the whole site.
Rich C.
P.s. I guess I should have refreshed the page before posting, computer was sitting idle with page loaded for a few hours, and look a dozen or so replys which I duplicated, Sorry...