Each of the three phases can supply a 120V single phase load between the line for that phase and neutral. If all you have is 120V loads, it is simple a matter of balancing the loads so that each phase has (approximately) the same load.
If you need to power 240V single phase loads, there may be several options. Some 240V loads will function quite well at 208V (read the label on the load to know); if that works you get 208V between any two phases. Again, you need to balance the loads across the phases.
Some generators can be rewired for 120/240V operation - although they typically can produce 2/3 the power they can produce on 3 phase. If the generator only has 3 or 4 leads, then it probably cannot be re-wired. If it has 12 leads, then it most likely can be re-wired. Check the generator data plate to see if it has a designation for the number of leads. To properly re-wire it, you'll need instructions for that particular generator. For a generic idea of how it can be done, look up zig-zag or double delta; those are the names given to two different methods of re-wiring the generator. Post pictures and/or the complete model numbers and someone here might be familiar with the generator to know how it can be re-wired.
