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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.
That makes sense - when the '3' appeared in both docs it threw me a little!