That link I posted has diagrams including an example of a DPDT in action. Well, DPDTs are really the on-on variety... but when you look around you'll see on-off-on and on-on-on also called DPDTs, they're really DP3T .
Using a simple example of a pickup selector for 2 humbuckers, an on-on-on switch would provide the normal operation that your probably used to (bridge-both-neck.) An on-on would be bridge-neck and an on-off-on would be bridge-none-neck.