Laptop + Docking Station Multiple Monitors

I have a Dell E6430 with a DELL PRO3X Docking Station. I'm trying to connect 2 ViewSonic VA2451M-LED monitors to it. I've tried: 1) One through the VGA port and one through the DVI 2) One through the VGA port and one using a Display Port Connector I can extend my desktop (laptop monitor) to either of these monitors, but it won't let me extend to both of them so that I can have 3 monitors at the same time. I've seen this same question on forums and folks have figured out how to do it, but no one has posted details on what they did. Any help? Thanks, Eric