Hi,
my dog has had diahrea for 3 days now and i was wondering if it is normal if she maybe just ate something bad?

Diarrhoea is never normal. If it lasts for more than 24 hours visit the vet. You'll probably be advised to give her no food for 24 hours then slowly (over a period of days) re-introduce bland food.

But a vet's visit is vital because after this long she will be getting very dehydrated which is very bad for her kidneys.

No, it's not 'normal'.  A dog can have diarrhea for a number of reasons, especially during a food change, but 3 days is excessive to say the least.  Have a go at the vet.