Your V3 MicroSquirt® controller uses a variable voltage signal from a throttle position sensor (TPS) to determine when the engine is at or near full throttle (to shut off feedback from the O2 sensor), when the engine throttle is opening or closing rapidly (and needing an accel/decel enrichment), and when the engine is flooded and needs to be cleared. Some people have managed to make their engines function reasonably well without a TPS.
You will need a TPS that is a potentiometer and not a switch. A potentiometer gives a rising voltage signal that varies as the throttle is opened. Many older cars had idle or WOT position switches instead of a real TPS. A real TPS gives a continuously varying signal with changing throttle.
There are three wires on the external wiring schematic that go from your MicroSquirt® controller into the TPS sensor. These two MicroSquirt® wires are +5 Vref signal and a sense line. There is a third wire going to the signal ground. Assuming that you have a proper potentiometer TPS, then +5 Vref goes to one side of the pot, the other side goes to the signal ground wire and the TPS sensor line is hooked to the wiper.
To hook up your throttle position sensor (TPS), disconnect the TPS, and use a digital multi-meter. Switch it to measure resistance. The resistance between two of the connections will stay the same when the throttle is moved. Find those two - one will be the +5 Vref and the other a ground. The third is the sense wire to the MicroSquirt® controller. To figure out which wire is the +5 Vref and which is the ground, connect your meter to one of those two connections and the other to the TPS sense connection.
If you read a high resistance which gets lower as you open the throttle, the disconnected wire is the one which goes to ground, the other one which had the continuous resistance goes to the +5 Vref from the MicroSquirt® EFI Controller, and the remaining wire is the TPS sense wire.
You will need to calibrate your TPS when you first set up your MicroSquirt® controller, and anytime you load new code or adjust the throttle positioning (base idle speed, for example) after that.
To calibrate the TPS sensor you need to open TunerStudioMS with the engine not running, and under 'Tools→Calibrate TPS' you must:
Note that this sensor's calibration IS retained in the MSQ when you load new code and restore your settings by loading a previous MSQ file. You do not need to recalibrate this sensor, unlike some other sensors.