Multiple webbrowsers and multiple sessions

I want to create three webbrowsers in my application (vb.net)
I want them to log on the same site, but with different
logon/passwords.

it does not work, since on the server, only one session is opened for
my 3 webbrowsers.

Is there a way to make the server (php) create a new session for each
webbrowsers of my app ?

Thank you,

Guillaume
.