I've come across a spec for 1.5 pair cable and wondered if it's just three conductor cable. It would be nice to have this confirmed, as well as have an explanation for why it's called in such a roundabout manner. Is the word pair a specific terminology in this context which somehow conveys something "(un)shielded 3-conductor cable" does not?
It's 1 twisted pair for a differential signal such as RS-485 and a single wire for a ground connection between the transmitter and receiver(s). You can see an example with a bit more explanation on page 5 of this: https://www.ti.com/lit/an/slla292a/slla292a.pdf?ts=1606836400144