This question comes up every month or so. Chromis will swim together for awhile. My group of 5 did great for a little more than one year. I loved the way they hung out together. As they mature, they start to get more aggressive with each other. Suddenly one starts to get thin and then is gone. Then the next smallest starts getting picked on. It doesn't get enough food and it dies. And so on and so on until one is left. I have seen this in several tanks. Yes, there are exceptions and yes, maybe with larger numbers it doesn't happen as much. One of the aquariums in Australia that I visited had several dozen in a huge tank. They did fine. That said, I have never personally seen a tank with between 3-10 chromis that have been together for more than two years. Remember these fish should live greater than ten years.
I really wish that this was not the case because these are attractive, hardy fish that look great when swimming in a loose bunch. They truly remind me of several of the shallow reefs I dove at on the great barrier reef.