The 3-wire "switch" as you call it isn't a switch. It is a pressure sensor that feeds A/C system hi side pressure data to the 3800 PCM.
The OBD2 3800 PCM uses this sensor reading to determine how to control idle speeds, radiator fan operation, and A/C compressor clutch operation when the A/C system is turned on.
If you want the 3800 PCM to control A/C clutch operation (via the A/C clutch relay), you'll need to install one of these 3-wire sensors into the smaller of the two lines that runs between the compressor and Fiero body and wire it up to the PCM. The 3800 PCM will not enable A/C clutch/relay operation without having this sensor hooked up.
If you do not wish to put the A/C clutch under PCM control, that is fine so long as you have some form of compressor or A/C system over-pressure protection in place (with the 3-wire sensor hooked up and the A/C clutch under 3800 PCM control, the PCM protects the system from an over-pressure condition by monitoring A/C pressure sensor readings).
The stock Fiero A/C compressor should have a red switch on the back that 1 or 2 wires hook to. This is the over-pressure cut-out switch. It should cut ground (or power, depending on the type of switch it is) to the A/C clutch if an over-pressure condition exists.
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[This message has been edited by Darth Fiero (edited 11-22-2013).]