Does the 3 year old go to Daycare or to Preschool? If so, how about a personalized lunch bag? It is a zippered square lunch bag that is insulated and has the name on the top of the bag. We've had 2 of them for our Son, they are so cute and can fit inside a diaper bag or backpack. I have the name of a lady who did one of the lunchboxes for us. She has a website with other personalized/monogrammed items. You could also do a monogrammed cup/sippy cup. You can get the name on there or an initial put on there.
An ornament for Christmas would be neat (I know it isn't Christmas) with something she is interested in. This would be something she would have for later and would last a long time and could be passed on to her and when she has her own family. My husband and I have done pottery and we made a bunch of ornaments that are really cute- there's a heart, a fish, a horse, a dog and a butterfly. You can personalize it with a Sharpie and put her name on the front or back. We have others like a tree or a bell, however most of them are geared toward boys.
We will be taking pottery again in August and we are thinking of doing letters for hanging in a room or on the tree. If you are interested in this, let us know. We can also make whatever you are wanting for an ornament/wall hanging.
A cute picture frame with something along the lines of Sisters or Little Sister on it. You can paint/glaze picture frames at Van Grow or there is a place on Vickery (can't think of the name of it). Or you could buy one and then when the baby is born, you could put a picture of the baby sister in there and that could be a picture for her room that hangs on the wall or on a shelf/chest.
Does she have a piggy bank? Maybe get a book on Sisters or being a Big Sister?
Hope these ideas help!