Right so this is the kind of thread I live for. Three nights is long enough to rule out a one-off weird sound, something was clearly repeating a behaviour or at least the conditions were repeating.
What did the camera footage actually show? That's the bit you've left us hanging on.
I'd also want to know - was the breathing rhythmic or irregular? Irregular tends to lean toward pipes, old houses do weird things with airflow and it can genuinely sound like respiration. Rhythmic and patterned is harder to explain away.
I've had a similar thing at my old place in Stafford, turned out to be a gap behind the skirting board creating a kind of bellows effect when the temperature dropped at night. Took me ages to track down. Felt pretty daft in the end but I'd rather have that answer than the alternative tbh.
Post the footage though. That's what we're all here for.