In line I have either a SPDT or a DPDT switch (how do I tell, it has three leads but three positions one marked 1 the other marked 2 with the center position as the third).
SPDT = Single Pole, Double Throw
DPDT = Double Pole Double Throw
Single Pole: only one lead/wire is switched
Double Pole: Two poles are switched
Double Throw: It can be switched to one out of two contacts
Center position can be off or something else - like ground, if the switch metal is grounded.
What you have might be a SPDT with Center Off, but since it obviously connects to something, you probably have a Single Pole Triple Throw.
[...] once I turn the switch to the 'OFF' position it is only a matter of time before the 9V blows.
Measure the resistance between the pins and between each pin and the metal on the switch - do it for each switch position.
It sure sounds like you are shorting the battery when you put the switch in the middle position.
Could you post pictures and/or drawings of what you did?