3 Phase welder with single Phase plug?

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A year ago I bought an old AIRCO Phase ARC 500 MIG for what I thought was a good price. ($250.00) A former employee of the company I got it from said that they were all single phase welders. After looking at it better, it says in broad daylight "3 Phase", which I am not wired for. I put it up on eBay for sale, and someone told me it has a single phase plug on it.  I asked the local Airgas store about it and the guy said a 3 phase plug has 4 prongs. My plug has 3 prongs. The 3 wires are going to L1, L2, and L3 on the welder. Is this some sort of weird way they wired it? There appears to be no ground wire in the plug.My question: Is this indeed 3 phase?
Reply:Welll, if he finds an engine-drive with 3-phase output he may be able to run the welder.  But if he has an engine-drive, what does he need the DC welder for?  500 amps?  Youch, that's some ooomph.  Without 3-phase power to feed the beast, all you have is a big paperweight or boat anchor.Hmmm, maybe if you find an engine and rig it to a b-i-g 3-phase electric motor and use that as a generator to make the desired 3-phase power and then feed that to the Airco.  Let's see, 230V and 80 amps is roughly 25 hp.  Upsize the needed motor-generator to allow for inefficiencies, and you are up to a 30+ hp 3-phase motor needed.  Then you need to upsize the engine to drive the 30+ hp electric motor, so you are then up to 40+ hp.  Certainly it's do-able, but not necessarily all that cheap or easy.  And definately not lightweight or easily portable.Good thing the welder was only $250!    The best laid schemes ... Gang oft agley ...
Reply:Thanks for the answers. I guess I'll look up an equipment consignment auction place and git er out of here.