I'd say by and far, the EASIEST way is to hold down the trigger until you have fired three times and then let off.
Otherwise, you need a computerized (good solution) or timed (not so good of a solution) solid state switching system... like the extreme-fire mosfets.
You can physically put the mosfet wherever you want to, as long as you can run the wires to it.
Electrically, it will wire in between the battery and your trigger contacts and motor (makes it kind of a triangle instead of a single loop). They are somewhat of a pain to wire, but can be well worth it.