However you should be able to get it here - https://www.filepuma.com/download/avast_free_antivirus-2/versions/
It's actually difficult to find an offline installer for a specific version, as many sites simply provide a direct link to the latest version for differently named variants.
That's what I am learning. Seems an offline install for 21.3 is not around anywhere.
Yes it is a pain, and filepuma was one of the better ones retaining the offline installations, but their policy change is crazy as they might as well not offer any 21.x.xxxx versions as the online installation as that would require an internet connect connection and that would try to install the latest version.
Now I don't know if trying the 21.3 online version would work or not. I'm wondering if that would check to see if your system meets version requirements (SSE3).
The other option would be to try the last 20.9 version full off-line installation.
In both of the above suggestions you would probably need to disable program updates if you manage to get it installed, to try and block updates past that version.