What is this three-wire cable hanging from my ceiling?
If there was a black wire in there, I would say it was a telephone line. The fact that the crimps are on red and green supports that theory, as red/green are the first to be used.  (black/yellow were spares; they got a variety of uses such as Princess phone backlights, second phone lines and the like.)

It's also conceivable that it's doorbell or thermostat cable, or phone cable re-tasked for those purposes.  (phone wire is a little light for that).

It's definitely not AC power.