Wiring up a 3 way mini toggle switch to act as pickup selector switch is really easy. It gives the option of bridge pickup, neck on its own or, in the mid position, both together. All it takes is wiring some jumper wires side to side across the switch lugs of the switch. Then connect the hot wire from the neck pickup to one end of the switch. Next connect the hot wire from the bridge to other end of the switch. Lastly wire a hot out wire from the centre connections to the connector for the tip of the jack socket. That’s it, job done. Easy guide to Wiring a pickup selector switch.
Just make sure you use a 3 position Double Pole Double Throw ( DPDT ) on-on-on type toggle switch.
Only a small 6mm hole is needed to mount the switch through. You can usually find room to squeeze one in somewhere. These switches are also used as series, parallel, coil cut switches. This is explained here.
These types of switches were used often on vintage Japanese guitars from the 1980s