I had 4 chromis, and they killed off two and now I've had two that seem to be doing well for the last 4 months. I like them. I may try to add another at some point and if they kill that one off, then I'll leave them at two.
I claim no specific expertise and would not intentionally criticize your comment. But I believe the survivors have sent you a message that they don't care for or want to be with others of their species. Perhaps the most responsible thing to do would be to leave them at two and avoid another episode similar to the Long Island Tanked event recently reported. This gives reefers a bad reputation and hurts our hobby.