FAQ about SSO multi-site

During up-graduation from 5.0 to 5.5 ( we have vCenter in Linked Mode)

To install vCenter Single Sign-On in a multisite can we have 3 sites and 3 vcenter server if yes then how it will work .

1 Install SSO at primary site

2 Install SSO at secondary location BUT point it to primary instance

3 Install SSO at third location and point it to 1 or 2 site ??

Thanks