Are you certain that using 2 is fine? The product description page says all three are required. It may be that each PCIe plug powers a specific set of chips; the 3 PCIe connectors may not share a common power bus at all as in, say, the S3 design. Without knowing the actual S5+ design, or having an actual unit to look at, saying "2 is fine" is only speculation at this point unless something more is known than what bitmain has disclosed so far.

Bitmain could confirm this here, but they aren't posting much these days. If anyone already has a unit, they could also confirm whether using 2 out of the 3 connectors works and post here.