I think a real cows heart would be awesome. Also agree w/ beef soup bones, along with an intact bone, so you can show the insides. At age 3-5 you want to focus on stuff they can touch and feel, and talk to them as little as possible. I also think the physics aspects of medicine are very appealing to preschoolers -like how muscles get shorter when they contract, how blood gets from your toes to your heart, how lungs inflate, how food goes in and poop and pee come out. This is all very interesting, magical, weird stuff which you can demonstrate with balloons, rubber bands, aquarium tubing, their own bodies, etc.
I also read a piece the other day about how we should be teaching about the immune system as a "force field" rather than the soldier fighting the germ model we usually use with kids. It better describes our microflora + immune system. I think my son, Star Wars fan that he is, would get that.
You can ask your local public librarian to put together a set of picture books about doctors/medicine/anatomy for preschoolers. They are really great at this, do it all the time, and usually love to help, if you give them a couple of days. Then you can let the kids look at them for the rest of the day after you leave, collect them when you collect your kid (as long as that's ok with the teacher - libraries usually understand that their books are for kids, and won't freak out if a page gets torn).