Horses, like humans, vary widely in ear size, shape, and position. Some foals seem to have their eartips turn or curl backward. Usually this becomes less noticeable as the foal matures. Ear carriage and strength in newborn foals can be an indication of their health. Dysmature foals (those that are immature at birth but had a normal gestation length) often have floppy ears. Premature foals (those that are immature due to an abnormally short gestation length) often have very pliant ears which, if curled up manually, tend to remain in that position. Normally foals with these conditions have other medical problems that require veterinary assistance. Since your foal is already three weeks of age and you don't mention any other problems, his ear position is probably just the "way he is." But to be on the safe side, have your veterinarian check out the foal to make sure that his curled ears are not any reason for concern.