C.L.
My daughter is also 3 1/2 and has an occasional accident. If she wets the bed, I don't make a big deal about it. I just double sheet (and plastic sheet) the bed to make for easy change in the night, because when sleeping, it is truly "an accident". If she wets her pants during the day, I make her more responsible for it because I know that she had fair warning to get to the bathroom and didn't. I'll make her change her own clothes or (attempt) to clean up her own mess on the floor. We had a problem with her wanting to change clothes constantly and when we wouldn't let her, she'd wet her pants so she'd have to change. She did it at preschool a couple times and the last time I told her she'd have to go back in the infant class if she did it again, and it's never happend since. But under no circumstance will I let her put a pull-up on. Not after all this effort potty training. That's like a license to pee her pants. Also, no undies at bedtime (just jammies) worked really good for us too.
Good luck
C.