I'm currently building a 180g tank(72x24x24), I don't want to spend too much on lightning and was wondering if 3 AI prime HD would be sufficient to grow corals?
It would probably work for a softy dominant tank but you'd need to add more if you're interested in a mixed reef or SPS dominant system.
BRS has a number of videos covering light spread and depth penetration with regard to the par and spectrum. They even cover the Prime HD. You can search for it on YouTube.