This is the 335th most frequent Finnish word.
noin
Approximately; about; around.
In this sentence, 'noin' is used as an adverb to mean 'approximately' and is used to estimate the distance walked without specifying an exact measurement.
Menimme noin kilometrin kävellen.
We walked approximately a kilometer.
Here, 'noin' is used to describe the manner of how something happened or occurred, and it carries an implication of demonstrating or indicating 'in that way'.
No siinä meni noin!
Well, it went about like that!
In this context, 'noin' functions as an adverb indicating an approximate time, giving the sense that the meeting begins at or within a short range of 3 PM.