I intend to purchase a new photopost classifieds liciense for a new site in which case I would get version 3, can I also use the script to upgrade my other 2 sites one under 2.5 another under 2.6, is it against the liciense agreement
That is indeed against the license agreement to use another license on another site where the members access is expired and your not entitled to run the new version.
You must renew members access to the licenses you wish to update the code on.