My first loss and should I keep 2 chromis?

I got 3 blue/green chromis last night. I debated the blue or the blue green and based on color I couldn't justify spending $25 each for the blue. Anyhow after acclimation two of them stayed together in the tank. The third stayed behind the rock and appeared to be breathing heavy. Parameters are good and the other fish are doing great.



This morning the third was MIA. I blew out the rocks to find him dead at the bottom. :(



I've read to keep these in odd numbers but the 2 I have seem to be happy together and stay together constantly. Should I add another or just keep the 2?



90 gallon, coral beauty, 2 clowns, one spot fox face, molly miller blenny.