What is your intake configuration? Do the three carbs feed a common plenum with lots of mixing, or is it more stratified flow? Also, is your center carb the one with idle/secondary function with the outers only having secondary like a "traditional" 3 carb setup? Teenagers like to "tune" - any chance the back carb got "tuned shut" at idle, making it run lean in back at WOT (probably at WOT a lot with a young driver...)?