Can you adapt 5 pin intercom headsets to work over a 3 pin clear-com system?
Please do NOT try to connect a headset directly to one of those wall-mounted 3-pin XLR connectors. Expensive smoke will be the result.

The headset connects to a box called a "belt pack". That belt pack connects the wall-mounted XLR connector.

Let's start with a bit about ClearCom and RTS Systems.

History of ClearCom Intercoms

History of RTS Intercom Systems

ClearCom systems of the vintage that you describe use a two-wire-plus-shield cable for interconnect.  One wire carries a DC voltage (nominal +24 to +32 Vdc) while the other wire carries unbalanced audio plus a DC signal for the call light on the belt packs.

The audio conductor is (AC) terminated at the power supply.  Different impedance values were used over the long history of ClearCom but were usually somewhere between 180R to 220R.  The reason for the AC coupling capacitor on the terminating resistor is to allow the packs to provide a DC voltage on the audio conductor if a pack initiates a call signal to the other packs on the line.  Audio level on the line is pretty near -20 dBu.

Older ClearCom belt packs are readily available from used Pro Audio suppliers as well as places like eBay.  These are easily brought back to usable condition by replacing volume control pots and, if necessary, the toggle and push-button switches.

ClearCom is generally used by theatre companies and smaller Pro Audio companies.  The main disadvantages to the ClearCom system are that only a single audio channel is carried on that 3-pin XLR and the packs are very prone to ground hum if the metal pack cases touch building ground (stair rails, electrical conduit, electrical cable trays, etc.  I speak from experience when I say that these ground problems can be very time-consuming to track down and eliminate.

The main advantages are that it is very widely used and it is relatively inexpensive.

RTS Intercom Systems are used for more professional user such as broadcast and very large show productions.  Their "TW" system uses the exact same 3-pin XLR as ClearCom but RTS fits both DC power and TWO channels of intercom down that one cable.  DC supply is about 32 Vdc and audio level on the line is pretty near to -10 dBu (1V P-P).  Line impedance is set at 200R at the power supply and both audio signals are unbalanced.

Ground hum problems are avoided by the use of fully-plastic cases on the belt packs and are reduced by 10dB should an accidental ground connection happen because of the higher audio level on the line.

Lots more information available via your favourite search engine.  You can also find schematic diagrams should you want to build your own belt packs.