How can i have a SimpleNews signup form as modal/lightbox
It's certainly possible, however, modals aren't as simple as clickkng a few things together (at least not before drupal 8 :p).

One way to do it would be to add a counter to user's Session. Increase the counter every time a new page is viewed and once it reaches 3, trigger the display of a modal, for instance by using ctools modal generation api.