Three nights in a row is not nothing. That kind of consistency is actually what separates a genuine encounter from a one-off noise you can explain away. Random creak in the night? Sure. Three nights, same thing, same location? That gets my attention.
First thing I'd want to know is what time it happened. A lot of cases I've looked into follow a pattern, usually between 2-4am, and if yours lines up with that it's worth documenting properly. Write down the time, how long it lasted, whether the sound had any rhythm to it.
Have you tried setting up a recorder? Even just your mobile propped up nearby. Not saying you'll catch something dramatic but having audio evidence is massive when you're trying to figure out what you're actually dealing with. Anyone else in the house noticed anything off lately, not just the breathing?