K.B.
The Terrible 2s have nothing on the 3s. At this point, she is old enough to understand good behavior, but may still have trouble with self-control in the moment. I think you need to go into these situations armed with tactics. Which means she needs to know beforehand how she is expected to behave, give her lots of options of how to handle things, and if she can't follow the rules then you have to leave. So practice what she should do if someone is playing with something she wants ("can I have a turn with that when you are done?" "Would you like to trade toys?"), what she should do with their answer (wait patiently, find something else to use), and finding other ways to express frustration besides screaming (come find you for soothing and deep breaths). These are challenging lessons for little ones (I have heard that "taking turns" is easier to learn than "sharing") but given enough practice and tools, she'll get it and your ears will get a rest. Hang in there!