At 350gms, they should be eating 96mls a day divided by the feedings. They should be getting 16mls if eating every 4 hours, at three weeks it can go to 24mls or so if every 6 hours. I would let them go at least 4 hours, maybe 5 or 6 and see if they eat more. Make sure they are warm too, a cold kitten will not eat, a cat heat pad would help, or mama, make sure they are huddling together.