I thought it may have been a typo in one of the KBs that I had seen (cannot remember which), but looking at DOC5762 the DAS50 is down as having 3 power supplies - p17 of ESM Installation Guide (9.3.0) shows 2 of them, is the other PSU around the front of the DAS50? Also, does it essentially require the use of all of them (I wouldnt say so as power consumption would indicate not) and if not, are the 'two spares' used as backup, with PSU2 being connected to a different circuit?
I don't have a DAS available to inspect, but I'm pretty sure there are only 2 PSUs on those boxes. I think the reference in the doc to 3 PSUs is a typo.
The 2 power supplies are provided for redundancy. Power supplies experience a lot of stress, and are one of the more common components to fail in any device in the data center (right behind hard drives). If you plug them both into the same circuit, then you have protection against a failed power supply. If you plug them both into different circuits, them you are also protected in case you lose upstream power in one.