If you are feeding fish that are only 3 days old, feed several times each day with whatever you find acceptable. The tiniest of fry can only eat so much at each meal but should receive food rather steadily. That will mean many small feedings each day. Small portions are indeed difficult to judge but you can always remove any uneaten food at 15 to 30 minutes after the feeding. That way the fry get plenty to eat but the water quality is maintained too.