At 3, she isn't going to have any real comprehension of what's going on.
Actually I don't agree with that. My sister was in hospital at 3 and still remembers it over 30 years later. I think you're absolutely right OP to prepare her as much as possible (without going OTT and worrying her of course!) As a nanny I've worked with little ones who've had operations. Here are some of my tips:
Yes to books! Ask at your local library but have a flip through any they suggest before you read them just to make sure they're age appropriate. This one is good:
www.amazon.co.uk/Going-Hospital-Usborne-First-Experiences/dp/0746066589/ref=sr_1_1?s=gateway&keywords=going+to+the+hospital+book&tag=mumsnetforu03-21&qid=1560298427&sr=8-1
I don't know this one but reviews say they've read it to their 3 year olds:
www.amazon.co.uk/Ollies-Tonsils-Tony-Densley/product-reviews/1925422186/ref=cm_cr_dp_d_show_all_btm?reviewerType=all_reviews&tag=mumsnetforu03-21&ie=UTF8
Watch video clips together. There are lots on Youtube. Again watch them alone first to see if they're suitable! It doesn't have to be the same hospital she'll be going to, it's just to introduce the idea. this one is good:
So are these - although aimed at slightly older children I think still worth a watch:
Role play is really good to prepare. I have a collection of real (but safe, of course!) medical equipment that I use with the little ones I work with. Toy doctors kits are better than nothing but don't bear much resemblance to the real thing. I have a real thermometer, stethoscope, blood pressure cuff and oxygen mask, and some extra clutter too. If you have any vet or doctor friend see if you can get any freebies from them - gloves, plastic syringes, scrubs, surgery masks and hats etc. The IKEA dolls bed or similar makes a great bed for a teddy or doll to be a patient in while you role play being the doctor or nurse, and your little girl can join in too.
Role play is more important at 3 than small world play, but that has its place too, so something like a Playmobil hospital might be good. If you were going to get her a present to take to hospital this is cute:
www.woodentoyshop.co.uk/mini-hospital-playset.html?gclid=Cj0KCQjwov3nBRDFARIsANgsdoHc2G_-kfsJqF7SoBd3ToDOAQMwPJSfCEHYbjyHZuyNE01TwrFyPJgaAuiGEALw_wcB
She'll probably role play hospitals for a week or 2 after the op to get it out of her system. Or she might not. Children often bounce through these things!