My first thought is that someone is playing a practical joke on you. I could understand the fake eggs and real eggs but all the hay too? No.
Pullets just starting to lay can do weird things but I would not expect them. Sometimes a rooster or cockerel may try to help a pullet or hen find a good nest. If it is any of them I'd suspect him. But that doesn't sound right either, not even with an adolescent only 21 weeks old. If he moved all the fake eggs that may have influenced the others to all lay there.
Could you post some photos of your nests and how they are arranged. Maybe we could see something. Or maybe the hay was not all moved to the third nest but most was on the coop floor? Chickens can do some really strange things but this doesn't sound like chickens to me.