Tracking of where leads come from

Hi! I'm using Wordpress + Constant Contact's Wordpress Plugin. I have a form that is generated by the plugin shown on a landing page on Wordpress. I want to be able to have multiple landing pages all funneled into the same Constant Contact List, but want to be able to track where each lead came from. I.e. If I have Landing Page (LP) 1, LP 2, and LP 3, and send them all to LIST 1, how do I look at LIST 1 and know if a lead came from LP 1, LP 2, or LP 3. Ideally I would also like to see analytics on how many leads came from each LP source. What do I need to do to make this happen?