Hi M.-
I suggest not talking about it until about a week before you need to check in. 3 year olds do not understand tomorrow vs 5 weeks from tomorrow, so talking about it today will just worry her.
I would start reading books about hospitals, how they make people better and other positive stories. If the hospital has a video of where you'll be, then you may want to allow her to watch it.
The maternity units at our local hospitals have short on line videos about what to expect, and your daughter may like it. If there is not a video, maybe tour the hospital with your daughter.
Do you have a lap top & web cam? I use Skype for many of my Parent Coaching sessions, and Skype is great. I you can bring a lap top, you & your daughter could talk every night via Skype.
Good Luck with your hospitalization.
R. Magby