It's about 30 atoms, depending on the material. Silicon's covalent radius is 111 pm, so it's 27 silicon atoms. But the 3nm process node doesn't mean that all the features are 3nm wide; a lot of things are twice that wide in many processes. Sounds like 3nm is pretty weird, though, so all bets are off.

IBM fabricated single-atom transistors that worked with adequate reliability back in the 1990s, IIRC. I don't know how bad the noise problem you allude to really is.