Most likely, the three-prong plug will have a red (hot), black (hot) and white (neutral). The neutral may also double as a safety ground which is the Bad Thing™ that the newer four-prong plug is meant to address. It will probably have the red, black, white and also a green ground wire. If this is the case, the best thing to do would be to have the three-prong outlet changed over to three-prong outlets. In the interim, you can get away with wiring a three prong plug to the the four-wire cords, leaving off the ground–but be aware that in addition to being less than ideally safe, this may also cause equipment malfunction if the ground is expected by the circuitry. I do not recommend you let that go for long, though, in the interest of safety (and compliance to the NEC). If there is some other combination of wires in either cord, the above will not apply.
Or you can wire the three prong to a four and bring the green safety ground out of the four as a pigtail which should be attached to the frame of the cabinet (I’m losing it-I had to look up the spelling of cabinet). Always assuming that the lack of a green ground is the reason for three prong plugs in the first place.
Still here, didn’t even bother trying it b/c when we looked at the new UPS, the receptacles in the rear for output are rated at 220V instead of 110V so the whole project would have been done in vain. And thanks to McKesson, the even grander project has been put off until April anyway. Thanks for the replies though, i’ll keep this thread handy.