I would probably not try three. My (limited) experience is they will kill each other til one is left. I was thinking of adding a second one recently, but changed my mind after reading a little and taking into account my own experiences.
I do not think you need to provide zoas for them to eat. Or coralline algae for that matter. Mine seems to do fine on a pretty much steady diet of new Life Spectrum pellets and daily sheets of Nori. (Although it does eat zoas if they are there) They need to eat frequently, though. I feed my tank twice a day, but especially when I first got the Idol, it seemed to want to eat more often.
Why would you leave the lights off?
I have mine in a community tank. With it I have an Asfur Angel, a Majestic Angel, a Purple tang, an Achilles tang, a Blue Devil, a Scooter Blennie, an Indian Trigger and a Mandarin.
I have a 240 gallon tank. I recommend a big tank, the Idol likes to swim.