I'm looking for new headsets for an old Clear-com intercom system in a community theatre. All the plugs in the walls for the system are 3-pin XLR, but shopping around I can't find anything but 4 and 5 pin XLR connectors on headsets. So my question is, what's the difference between the three different plugs in an intercom system? If I were to invest in 4 or 5 pin headsets, would I be able to adapt them to work with the 3 pin plugs using plain 3-5 pin adapters? Does the 5 pin plug actually have 5 conductors or does it only have 4?
3-pin is usually for balanced mono lines - a single balanced mic, etc.
4-pin can carry a mono signal to a headset & a mono unbalanced mic return.
5-pin can carry stereo to a headset & a mono unbalanced mic return.
6-pin can carry stereo to a headset & a mono balanced mic return.
If you try to reduce a 5 or 6 pin setup to run over 3-pin, basically all you could do is carry a stereo signal to the headset, with no return - or you could attempt to use one pin as ground for both mono signal & mic return, but I'm not sure how happy it would be doing that. I wouldn't risk it.
Ref: Wikipedia - XLR connector
XLR-3 connectors for a Clear-Com intercom system might be for analog partyline systems. And actually, analog partyline systems are (or used to be) quite common in theatres.
If this is the case, it means you cannot directly plug a headset on these plugs, you need wired beltpacks on which you can plug a 4 or 5 pins headset.
The OP mentions that the main unit is a Clear-Com PS-451 which is actually a power supply and a main unit for such two-wires systems.
If you want to use this intercom system, you have to check wether currently available beltpacks would work with this main unit, and then ask for a quotation from a Clear-Com distributor (that might be the painful moment as these are usually not cheap). Or try to find some used ones.