How can I create single sign on across multiple single site installations?

Hi guys

Can anyone help with the following, I’ve read many posts about this but getting nowhere!

I have three single site installs, all on separate servers but are sub-domains:

I have as the parent site for users and have the User Synchronization plugin installed on all sites. What I want is the users to be able to login to one and be automatically logged-in to the others.

These can’t be converted into multi-site due to plugin incompatibility neither can they be migrated to one server to share the DB user table.

Any help appreciated.