How do I know if my 3 week old foal is getting enough milk from the mare and how soon can I feed it?

If the foal looks healthy, bright and active, then it is probably getting adequate nutrition. 

If a foal is not getting enough, you will notice a lower body condition (less than 4 on a scale of 1 to 9), plus you may see the foal continually trying to nurse. However, remember that a foal normally nurses several times per hour. A typical foal will nurse, maybe play some and then nap.  Upon waking, the foal will stretch, then either interact with its environment or nurse.

The best way to evaluate your foal’s weight is to weigh the foal every day or every other day. A foal should gain 1 to 2 pounds per day. If you do not have a scale, (weight tapes are not accurate at such low body weights) contacting your veterinarian may be the best way to determine if the foal is at a healthy body weight.  Also, having a person experienced with foals, like your veterinarian, observe the foal is acceptable. Monitoring your foal is essential as a foal can go downhill very quickly without enough nourishment.

With regards to when you can feed your foal, a foal will start to mimic the mare within a day or two of birth.  However, the foal will not start to eat any amount until about 2 weeks of age. If the foal is hungry, it will start to consume feed sooner and eat more than a foal that is getting plenty of milk.  There are a number of foal feeders/creep feeders available to allow supplementation of the foal while with the mare.