Hi,
from my experience, it is normal that your piranhas eat like crazy because they are still juvenile. At 3 inches, they eat 3 times a day... so mutiply this by 12... that's a lot of food.
By the way, your piranhas are still juveniles so it is the time to introduce the pellets.... in the end, when they'll be hungry, they will eat them and they will get used.
for the firt two months, i fed my RBP only with breeders... when I introduced the pellets, they didn't eat for 2 days... but they got used and now they only eat that and they have stunning red bellies since pellets are full of pigments which bring the colors out.