Frogs don't understand the concept of sharing, it's survival of the fittest. Whoever catches the most food wins, if you want them to all get a fair amount your best bet would be to separate them for feeding. Perhaps feed them one at a time in a separate tub/tank. That's your best bet to keep them evenly fed. You could try putting food in front of each frog and make sure no one else takes it, making sure that the frog that the food is meant for gets it.