What does it mean when caller id only displays three digits and nothing else?

Not sure, but when a call comes through as unavailable I don’t answer & say to myself, “yes, yes I am”

I have never see a three digit number make calls but I have a feeling it’s just a scam.

It is a way for scammers to get around some call blocking features. A simple call blocker will not ring your phone if the ID is blank, Private, or Unavailable. By putting in some digits they pass that hurdle.
Don’t answer. If it is important they will leave a message.

Lots of ways to “fake” caller ID – depends on the way that the caller’s settings (or switchboard) was set up. Some VOIP providers even give their customers the ability to “spoof” callerID. Not for nefarious purposes, but rather as a convenience.

So either someone screwed p their personal settings by accident, or (as others have said) it may have been intentionally someone trying to do telemarketing.
In any event, ignore it.

I get weird numbers on my CallerID all the time. I just ignore the calls.

Sometimes the odd numbers can be legit. When my husband was deployed overseas, a 4 digit number would display on my caller ID when he called. It was just how the phone system was set up.