Hi...
Yes, you can use avast on the other two computers but I'm not sure precisely if you can use the same license for all three. I'm pretty sure if we were talking about the professional version, it would be a definate "NO" but I'm not sure about the home version. As long as all three copies have access to the internet from your network, they should update just fine.
Also, if I understood correctly, you're running two antivirus together on the same computer. This is
never a good idea as AV programs, by nature, are "territorial" and running two together can cause conflicts and other problems. For this system, you should choose one AV or the other and uninstall the one you have not chosen.
I would need someone else on the forum to help with the license question...
Best Regards...