This confused me too at first before I did some research into three phase. I like to think of it like this (and please pardon my newb explanations):

Single phase AC has two hots, with each hot maximum potential 180degrees out of phase each cycle(hertz)

Three phase AC has three hots, with each hot maximum potential 120 degrees out of phase each cycle. Think of this like a triangle instead of a teeter totter.

You can use Three phase as single phase, just use two hots from the three phase. Of course this will only put a load on one 'side' of the three-phase triangle. The additional trick needed for three phase is 'balancing' the load of all three 'sides/phases' of that triangle so power is pulled evenly. Ammeters on each phase can provide a visual representation of what is going on, or you can just depend on careful calculation and wiring. Transformer life and many other thinks can quickly deteriorate if you are running unbalanced in any significant way.

So with just one more hot with 3 phase, you essentially can pull single phase from three points.

An example with a 3 phase pdu I have: 3 phase power cord, 60A, @ 208V. It has 6 C-19 outlets on it. Outlets 1-2 are connected to Phases A-B. Outlets 3-4 are connected to phases B-C. Outlets 5-6 are connected to phases C-A. It will not self balance, but requires that similar loads are put on these groups of outlets.

Here is a number you should memorize (it is not exact but gets you close enough)

1.732

To calculate wattage available from 3 phase : AMPS X 1.732 X VOLTAGE

Hope some of this helps, three phase is cool beans!