When I was little it was the same scenario. I see and feel spirits, my parents ignored it and I believed I was a little crazy. However and that being said, years later I was able to discuss these things with my mother who said she knew something was going on. The point is, she could have told me back then that I wasn't crazy
I would definitely talk to your three year old but in a way that makes everything that's happening normal. I wouldn't get elaborate on her just keep it simple. When you ask her about what they want or what they talk to her about, let her know (like many others have said) that she can ask them to leave her alone if she wants.
She will be fine with your support.