Dear M.,
My daughter turned 4 in June and she has only achieved night-time dryness 90% of the time in the last two months. She'll go for about two weeks keeping her pull-up dry, then have an accident in the middle of the night. I was very frustrated trying to get her to this stage, and I looked up bedwetting (for lack of a better term0 on the internet and got some great information from childdevelopmentinfo.com.
My mother's solution was to keep me dehydrated. I swear she gave me about 9 ounces of fluid a day. My glass was the size of a shot glass, and I don't remember drinking all that often. I refuse to use this technique because of health reasons and because I think it is cruel.
You might want to switch to a pull-up rather than using a diaper. Only you can decide if you want to use a pull-up (my choice because it is easier) or let him wet the bed (I don't want to wash sheets and waterproof mattress covers everyday). Just keep in mind he's only three and I don't know how much he can help you change the bed everyday.
What has been working for my daughter is fluids restricted after dinner (but if she has been running around getting hot, I let her have a small drink), and doing "night-lifting", I take her potty before I go to bed, and her father takes her when he wakes up in the morning--otherwise she is up at 6:00 am to go potty and that is too early for me. I think she has too issues that obstruct her goal of nighttime dryness on her own: being a sound sleeper and heredity. Scientist have done studies that some kids are very sound sleepers and just can't wake up when their bladders are full. And this has nothing to do with the deep sleep that people with sleep apnea have. Of course, you need to decide what you are grateful for. I am very grateful that my daughter is a good sleeper and she'll get better night-time dryness eventually. Also, her aunt and one of her cousins wet the bed until they were seven, so I'm wondering if this tendency came from her grandmother (though she denies it). As far as I can tell, their mothers did nothing to help them stay dry at night. Most doctors don't consider bedwetting or not be able stay dry all night a problem until the child is 6 or 7, and I'm disappointed to learn that the hormone therapy isn't that effective.
To help your son during the day (since he seems to be still "hit or miss") you might let him watch a potty video such as "Potty Power"; my daughter still consults her copy on occasion. And this sounds yucky, but put a potty in his playroom. That way when he needs to go he won't have to stop playing for so long. A friend of mine did this with her daughter. She was four before she was potty trained and it was because she was too busy playing or watching TV to go use the toilet.
Hope this helps, and don't get discouraged. Boys take longer than girls.
A.