Is she being prompted? It’s weird that 3 year old randomly brings this up at a party.

I think the only way to prevent such things happening is not to be with people like him

It’s no matter what you do, he’ll do what he wants by escalating conflicts and then blaming you. Didn’t it happen before? Different incident? Again it was with the door and one of you holding a kid and you trying to get him to leave and him yelling that you are abusing him etc It’s not the first time. You can’t stop this from happening and your kid from telling things to people. It’s just how it is going to be in relationship like yours. Very unsettling