I needed one squeak. But the woman at the hardware store told me they only sold them in packs of three.
So: the first one will go, as planned, at the top of the stairs. In the hours of darkness, it will tell me when I've arrived at the top step. I will then reach for the handrail and continue my journey down to the kitchen without turning on a light.
I've decided to put the second squeak outside the bathroom door, as a kind of signal. Until I find a place for it, the third squeak will stay in the package, to keep it fresh.