Three nights in a row is not nothing. That's a pattern and patterns matter way more than a one-off thing you can dismiss as tiredness or the house settling or whatever.
First thing I'd actually want to know - did it sound like it had a rhythm? Because shadow entities and certain types of presences tend to produce irregular sounds, kind of mimicking breath but not quite nailing the timing. Regular breathing with a normal rhythm would lean me more toward an animal getting in somehow, even if you can't figure out how.
Also where are you based? Some locations just have more activity than others, I've noticed that living up in Inverness.
Has anything new come into the house recently? New object, bit of furniture, anything second hand? Asking because haunted objects are massively underestimated as a cause for this kind of thing and most people don't even consider it. Three consecutive nights really does suggest something has attached itself to the space rather than just passing through. Anyone else had similar and tracked down a trigger?