Those are 3 of which I'm familiar. I frequent COTH and EC, but stay away from ABR. Not because the folks over there are mean, but because I don't have the same interests they do, and many of them are just plain crazy.
There are tons of other horse BBs out there, some moderated, some not. I prefer the moderated ones, since without moderation people can get downright homicidal to each other.
Just do a search for equine bulletin boards. You should be able to find one that suits your needs and interests.