First of all, you must understand that you will not achieve anything if you make she play with other children.
She is already 3, she can talk a bit. Try asking why she does not want to share her toys.
Best of all, if you show her that playing with someone is much more fun. Play with her, build castles, come up with stories, make her interested in playing with you and other people.
In the end, there is nothing terrible in that she does not share, this may be a feature of her character. You must treat this with understanding.
Every time she is near other children, try to kindly explain to her that the toys will be in safe if she shares them, and kids will definitely turn them back