It is not necessary that you need to point all virtual centers to same sso server. You can deploy sso server at all the sites.
For Ex.
- Deploy first SSO-1 server on site-1
- Install virtual center server on site-1 and register it to SSO-1
-Deploy second SSO-2 server on site-2 but while deploying second sso server select the third option , click next and provide the first sso details (it is called multi site).
- Install second virtual center server on site-2 and register it to SSO-2
- Deploy third SSO-2 server on site-3 but while deploying third sso server select the third option , click next and provide the first or second sso details (it is called multi site).
- Install third virtual center server on site-3 and register it to SSO-3
- Now you can join virtual center server from first to second once it is done then join second virtaul center server to third virtual center server.
- If it configured successfully all three vc is linked.
Please let me know if you have any other problems/doubts.