I'm assuming your chassis has an Ablecom SP-762. It's about the only setup that uses 3 SP382-TS power supplies, and that's what Supermicro uses.
This is a power supply enclosure that holds three small power supplies, each with their own AC connector. It has a total output capacity of 760W. It can provide the full 760W even if one of its smaller power supplies fails, since each supply can output up to 380W.
There is nothing to equal 1140W. The power supply cannot put out 1140W. At any time, the power supply can produce up to 760W, so long as at least two of its modules are working properly.
All are running at once, with one having the ability to die leaving the server with 760W. No power is wasted. These PSUs are specifically designed to operate in this way. (PSUs are typically designed for 70% load because that's what they typically see. Redundant power supplies are an exception and are designed appropriately.)
This review on Tom's hardware shows the power supply.