Login Problem

Hi
I am Partha. I am developing a site using wordpress 2.7.1.
There i am using three installation for different purpose.
All tables for these three installation in one database. I also integrate the user tables for these three. But i am unable to solve log in problem. I want to make like that if logged in/out in one blog then i have no need to again logged in/out in another blog.
It is very urgent to me. I read so many posts here but didn’t get any solution. Please help me ..