It's your first 3 days, everything is going to be confusing, and nobody should expect anything more of you! I can't speak for your store, but I've been working at McDonald's for 6 months and, even though I have had plenty of actual shifts, I am still regarded as a "trainee" because it takes time to pick things up! Nobody expects perfection, and even the customers can be quite understanding! Everybody is slow at the start, and you work your way up! I think a change of attitude could help you enjoy it, if you could just see that nobody expects you to be as good as you think you have to be!

If you really do hate it though, quit. You shouldn't have to be stuck in a job that makes you depressed. Would it look bad quitting just after training? Well, yes, but these are people you never have to see again, but if you really do care what they think, explain how it's not what you expected and cannot manage the stress. You will get paid, but don't think 3 days is something to put on your resume. I would probably show up tomorrow, because it is an inconvenience, especially with such short notice, but if the negative mental health you would get by being there drastically outweighs the negatives of not going, maybe you shouldn't go. If you are a casual, I believe you can terminate your working there any time, but if you're part-time or full-time, I think you need to hand in a two-weeks notice.

Good luck!