I've noticed that among many triple-boarders they don't know child psych, or general psych, or pediatrics as well as someone that just did one thing.
E.g. they couldn't run a psych ER as well, an inpatient unit as well, or do pediatrics as well.
Now this wasn't all of them, just most of them. Further they had a majorly frustrating position of having to take 3 board exams! Three!
I'm triple-boarded but one of those boards is one where I didn't have to do anything. It was just that I had to show I did some journal articles and pay some money. One is in forensic psychiatry and the other is in general psychiatry. IMHO those two actually work together cause inpatient and forensic work quite well especially when you're identifying and kicking out malingerers or involuntarily committing them.
Another frustration is the way the field is, it's easy to get a job where you're using one of those specialties but not all three. Trying to do a job that does all three is kind of like jamming a square peg into a round hole.
For which jobs would triple board be necessary?
I can't think of any. The problem is there's so few of them and a niche where all three specialties are needed in one place aren't exactly compatible with the way hospitals and outpatient offices arrange themselves.
It's kind of like setting up a restaurant at a place where you change your oil and eat a meal while your oil's being changed If it's not finished by then you could watch a movie. "We're looking for mechanics that went to culinary school but can also run a movie projector."
A strength, however, of the triple-board is sometimes these fields do have a grey area in between all of them such as, for example, child custody cases where there is physical abuse, physical problems in a child with also psychiatric issues, etc, and when this happens a triple-boarder (at least from what I've seen) is invaluable.
If you are very interested in all three fields, and see yourself somehow using all of them go for it but don't just do it to one-up everyone else.