Normal, no but it does happen. Your clowns have formed a pair bond against the odds man. Lots of times when you have two different clown species in a tank you can have major problems but, you didn't so don't worry. The maroon is the girl and the percula is the boy. I wouldn't dare add any other clowns to the tank though.
lol I don't hear that happening often. You should post some pics I definitely wouldn't add anymore clownfish. You might upset what little balance they have. You will probably have to keep a closer watch on them then a normal pair.