Avast Free 18.3.3860.0?

I am confused - I have downloaded Avast Free offline installer and it shows file version 18.3.3860.0. Is this the latest version?

Yes, that is build number.

https://imgur.com/a/dL4pdF1

The latest build number is 18.3.3860.316 which was a micro update (probably a bug fix).