Poop eating dogs?

ok so i have 3 dogs. 2 of which eat their own stool and eachothers stool. only one will not eat stool, but enough time pes watching the others she might do it too. i tried for-bid and deter; sorta worked but not really. i do keep their poop picked up but this is my problem... with 1 of my dogs he seriously watches the other dogs and when he sees them squat he speeds over there and seriously eats the poop mid air. obviously its not boredom or cleanliness. i have read maybe its their diet, i feed nutro natural choice. it might not be the best in the whole wide world but it is a premium food up there with royal canin and science diet, i doubt they're lacking nutrients. i doubt a switch in their diet will have any effect if forbid and deter arnt working. i did sprinkle chili powder on some stools and they wont eat those ones, but they're smart and they know that the stool straight from the source will not be tainted w/chili powder. i dont know what to do.