asp.net application integration

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ASP.NET

Hi,

I have three different web application with login pages, A user may active in three applications and have three different credentials
Now my requirements is :
for Exmaple :
if a user 'A' logged into 'app1' using his credentials ,then he can navigate to other two application without credentials

its simply like gmail ,if we logged into gmail then we can able to access drive,search without credentials

i need most secure method ,my application under vulnerability testing so query string should not be used

thanks in advance