Have you asked her why all of a sudden she is having this problem? I found that by age 3, I was able to talk to my kids and ask them questions. On something like this, she might be able to help you understand what is causing it. I.e., a PP suggested UTI. She won't be able to tell you that but she might be able to indicate that either she needs to pee all the time or it hurts (burns) to pee, or something that will give you a clue that this is the cause. Otherwise, if she tells you just "forgot", it could indicate she is being a normal 3-year-old who gets so busy playing she leaves it to the last minute to go and then doesn't quite make it, or it could mean that she is feeling insecure/angry about how things have been lately, and taking control of the one thing she CAN control. In any case, I would try to have the conversation with her, and see if she can clue you in as to what is happening with her. If you wind up feeling it is behavioural, I would suggest a positive reward system. I.e. if she goes half a day with no accident, she gets a treat (worked out between the two of you in advance). After a few days, up the ante to no accident all day, then 2 days, then a week, and so on. This will help her feel in control again and will be a win-win for both of you.