The load is shared on all 3 power supplies, but one is for redundancy. When one drops, you have 380+380=760Watt hence the rating. Its designed to keep running when one drops, that's why they don't rate it for 1140W.
On a dual P/S configuration of the same power, they would include 2x 760W PSU's. Again the load is shared until one fails. But since its designed to keep going after the failure, its rated for 760W not 1520W.
The-Wabbit, is spot on about how loading and efficiency is different on a dual vs triple PSU. PSU's run under 20% are not very efficient. (newest titanium PSU's are nice however)
Say your system idles at 250W and peaks at 450W on your 760W rated system.
Dual power supplies sharing the load, your just under 16.5% load at idle, not good at all for efficiency (250/1520).
Triple PSU's sharing the load, it idles at 22% which is a better number.
And in either configuration when one PSU fails you still have 760W of rating.
However their are some benefits to having dual vs. triple. With dual, you can easily run them on two UPS backups, on two circuits, on two phases. Each decreasing chances of the system going down, but attempting to do that with triple is a bit much.