Using different sub domains and servers


I am developing project which uses three two subdomains, and a main domain, either on different servers entirely, or a multi-site setup.




All the signup forms are going to be on #3, the actual payment gateway and user information is on #1. #2 is the actual service and product of the project.

I think it is better to separate the different parts as such, completely separate from each other. My question is, how can site #2 validate a user login that exists on site #1. Perhaps like a license server.

Thank you.