Definitely baby wear, that is probably a number one must do/have.
You'll get to know the place, so you'll know what she can get into...another must do
Come prepared with fun crafts and stuff to do, here are some sites you can pull some ideas from.
3hrs is a good chunk of time, enough time to read some books, do some crafts have snack etc. You'll do just fine once you find your routine and get to know her. GL