I've bred several different varieties of comps and calvus. Their favorite spawning cave (for me) have been the large whelk shells I have. The females can fit in, and block the entrance so that the males or other fish can't get in.
I feed my comps/calvus a normal meateater's diet, supplemented with blackworms and other tasty treats. In my case, the males picked one female to spawn with, and seemingly ignored the other(s) in the tank. Other people use other types of caves, but the main idea is that it should be narrow, to help the female guard the entrance, both before and after the eggs hatch!
Best of luck! I have a non-producing trio myself at the moment... need to feed them up a bit, I think!