They don't make an adapter that I'm aware of. Additionally, given that XL lenses were designed to cover a very small sensor (1/3 inch or so), there's no way the lens will cover your full-frame 5DIII sensor. Even if you could adapt the XL lens I'm not sure why you would want to. XL lenses are cheap, not sharp, and won't cover your entire sensor.
The only reason I was interested in doing this was to save some money initially - get the camera body first, use the old lenses, then buy better lenses later. After some unsuccessful searching to find an adaptor I suspected that it was not possible for one reason or another to adapt the lens. So... guess I'll be on the hook for a lens too.