P.S.
I think it's a stage.
In the meantime, provide lots of easy share toys like LEGOS, coloring books and crayons & markers, (washable), in lots o colors, Play-Dough, etc. When she has a fit over sharing just keep on her about how this is the way it is. "We share with our friends and take turns." Re-assure her that she will get the item back and make sure she gets it after an appropriate time has passed. Practice and set examples with lots of 'May I please have ____", and do a trade for stuff. Keep with it and give her time. P.