If a spoke loosens that fast then it's either very low tension over all -- low spoke count wheels need higher tension than 32h wheels.

OTOH it could be as simple as a stripped nipple, though usually these creak and pop like crazy when you try to true. Ar this point I'd remove and replace the, painting the spoke threads with something that's reduce the nipples ability to spin freely. On newly built wheels I use a very sticky grease, but in a pinch you can use beeswax, (Burt's Bees lip balm) rubber cement, latex paint, a dab or nail polish or bike touch up paint (don't over apply), or even paste car wax. What your looking for is anything that will let the nipple turn, but is sticky enough to keep it in place when there's no load.

At some point, (sooner, rather than later) I'd properly retension and align the entire wheel. Odds are you hit a pothole which brought a section of rim in a bit, so those three spokes can't be tightened as much without creating a low spot. That can be managed but there will always be a zone of lower tension there.