1 . Ground
2 . Constant Power supply
3 . Power supply to bulbs to 3rd prong.
Ground is activated by the combination switch in vehicles but if you wish to use it for other applications, you can install a switch either before the bulbs or along the ground wire to open and close the circuit.
Flasher relays are specified for certain amount of current and number of bulbs. This must all add up correctly failing which it would not flash but only remain on or flashes too fast.
Here is a wiring schematic of a typical flasher unit. It should alow you to understand the circuit.