It usually takes a little time before the change is reflected in the my.avast.com page as there is a setting as to how frequently the account is updated on the status of your devices.
You copy the license.avastlic file to the second computer and apply it in the same way as you did for the upgrade from Pro to AIS.
The third license is available for use in the same way, but be aware the time starts ticking from purchase not from when you first apply the license to an installation.