Well the last email:
Subject: Download link and license for "avast! Internet Security 5.0 (3 PCs, 2 years)
This should give a link to download the license file that we are talking about.
But you must have done this once or you couldn't have entered it in the first system, to do that you would have had to have saved that license file. Copy it to the other systems or try and download it again from that last email and repeat the process.
As I said, that same file is valid for use on the other two systems or however many licenses you have purchased (it is a multi user license).