Have you also placed a cover over the damp filter paper with the seed? If not, the seeds may actually be drying and this would keep them from germinating.
Also, I'm sure you realize that continuous light is not necessary (else there would be no lettuce in the world!). Lettuce seeds are also know to no germinate if they get too hot, so set your covered petri dish out of direct sunlight.
Lastly, seeds are not guaranteed to germinate within 3 days at 70F, so be patient. It might take as long as a couple of weeks. Just be sure the seeds remain damp and not in complete darkness.