Quit job before even starting

Has anyone done this? I recently accepted a 3rd job. Seemed like a good opportunity and in a great place. Since then I have decided my plate is too full as it is. And my child has some health issues going on with some appointments coming up. I would start next week. How bad is it to quit the job without starting yet? I think it might be better to let them know beforehand, then to start and then have to quit a few weeks in. Not sure which is worse. Thoughts?
It would definitely be worse to start the job and then quit a couple of weeks into it. If you quit now, at least they won't have spent any time or money training you. You could just tell them what you said here, that it was going to be a third job but that your child requires some unexpected extra care right now and taking on a third position isn't going to work after all, and that you felt that it would be better to tell them right away rather than let them spend time and money training you just to have you quit later. It might be an uncomfortable thing to do, but it will be a lot easier than quitting after they have invested something in you and if you quit now they may still have a chance at hiring one of the other applicants who would have been a good candidate for the position before they find something else.
Thanks. I did call back and say I couldn't take it. This was an area I have already worked in. I decided to follow my heart and either take only a dream job or not take anything new until then.