What could be causing my 3 year old to constantly grind his teeth?

Usually growth: Not much you can do about it, you must realize that many children do it. It is my estimation that it is a growth issue. No reason to seek care for the problem as there is no remedy.

See below: No one knows why bruxism happens. But in some cases, kids may grind because the top and bottom teeth aren't aligned properly. Others do it as a response to pain, such as an earache or teething. Kids might grind their teeth as a way to ease the pain, just as they might rub a sore muscle. Many kids outgrow these fairly common causes for grinding.

Stress?: One thing that can cause stress in a 3year old at night is not getting enough oxygen. Have your child's doctor check for allergies and other nasal blockages.

Normal : It is entirely normal for a child to grind his teeth, especially during sleep, but see the pediatrician to rule out reflux, sleep apnea, other causes.

Tooth grinding: If airway obstruction has been ruled out by pediatrician (enlarged tonsils/adenoids, sinus infections), see your child's Dentist (preferably a Pedodontist, a pediatric dental specialist) to rule out narrow upper jaw. If too narrow can affect growth & tooth eruption as well as breathing. For most kids, tooth grinding occurs as they try to adjust their occlusion and they "back up" their computers.