First of all, take the child to a doctor. Read up on the subject. It's been awhile since I read up on it, but I do remember it is a sleeping disorder.
I believe it is also hereditary. My Mom & her Mom, wet the bed until they were 12 years old. My eldest brother wet the bed until he was 14 & I wet until I was 12. My two younger brothers did not have this problem and they teased us until my Mom stopped them. It was very embarrassing. I have 3 children. Only one of them had a bed wetting problem. She was about 6 when she stopped.
My Mom would monitor the liquid intake of my brother & I after dinner. She made sure that we only had enough to wet our whistle. She would also wake us up in the middle of the night. I don't think it helped much....we would still wake up wet.
At three years old, I don't think I'd be too concerned about it. In a few more months. I don't think Mom ever took us to a doc about it. I didn't take my daughter either. I did read up on it though.
I wish you & your child luck. It is no fun being a bed wetter! What ever you do, do not punish the child for it. A person can not help what they do in their sleep.