That's the thing. He
doesn't want the middle on all the time. Just in the middle position.
"Microswitch" is actually a brand name, but it has become the generic term to refer to any very small switch of this general style. There's also Cherry-switch, Alco, and many others.
View attachment 105209
There are all kinds of actuators and styles. Those would fit on a penny. So, for guitar use, I usually have to fabricate a simple mount of some sort, (usually just a piece of hobby brass), and then position it so that the actuator gets "hit" by some portion of the regular switch when its in the right position. It's not as complicated as it sounds. It's just for those weird situations when you need one more contact.