I'd like to help but I have no idea how a "nerf gun" works so your description means little to me.
What is the specification of the motor you want to drive? Exactly what 3-"possession" (I guess you mean 3-position) switch do you have? Do you really mean that the 3 modes just drive a motor for different lengths of time?
Do you already have the trigger and the motor working with an Arduino? Which Arduino? Please show the wiring and the existing code.
Steve