[bread_baker]

Smoke detectors are sold individually. I'd say "each."

[eaturcheese]

There really isn't a unit of measurement needed with something like a smoke detector. I would probably say "three smoke detectors." "Unit" acts more like a pronoun, so rather than say "this smoke detector" you can say "this unit."

"Unit" is similar to "item," but I would say that the former applies to apartments and appliances, larger and more complex items, and the latter applies to smaller objects, like stationery or jewelry. If you are talking about shopping purchases, you can say "There are 5 items in the shopping cart."