By 3 young ones, you mean how long are they? Their daily diet must be at least a quarter of their body size. They are quite tolerant of you skipping giving food to them about every other day though but they are prone to overeating so watch out. It is illegal to feed fish in the UK if i'm not mistaken, so freeze-dried krill, shrimps, bloodworms are usually given. I've seen video documentaries of piranhas in the wild eating fallen overripe fruits so you could probably try that too. Feeding diced boiled chicken meat every other day is also nice.