Well, I have no experience with FBTs, but I do with American Toads, so my two cents would be:
Take "Fatty" out of his normal tank, and put him in a smaller tank where he can hide more easily, and does not have the stress of the other two (if he is sick).
Try a dry bath tub or something similar to that, where he can hop around in the light, on a bright surface, then feed him cricket by cricket, and see how much he eats, maybe try other foods, such as worms.
Monitor his food intake, and compare it to your other FBTs, from there you can judge if he's eating normal or not.
That's my input, sorry I don't know more, but I'm sure someone on this great forum will chime in, and help you out!
Best of luck!