At 3 she isn't a baby. Keep insisting. Ignore her crying. You are in charge. She isn't.

Before all of that, find out what has happened in the last few days that has changed, reassure her. What has happened at night is a reflection of what happens during the day. Children don't suddenly change their behaviour without a reason.

I know. I've got three, now in their teens. And they still talk to us!