Multiple Blogs, one login

I have 3 Blogs running on one IIS Server. Each Blog requires an Active Directory login.
– In the first Blog eceryone with an AD login has access
– In the other two Blogs only users who are member from a specified Group have Access

Right now I’ve got a startpage with a Link to every Blog and if you open a blog the login screen of the selected Blog appears.

What I want to have:
When you open the startpage it requires an Password. After entering Username and PW it should redirect you to a Site where you can see only the Blogs you have Access to. When the user selects one of the Blogs no further login appears.

Has anyone got an idea how to implement such a login to WordPress?