This recently happened in my flock. My broody sat on just three eggs. It was an afterthought to give her these eggs, and two had been refrigerated while one had been laid the day she began to sit.
Just one egg hatched on day 20. I gave it 24 more hours for the other two to hatch, then removed them. Until I removed the eggs, she was determined to keep sitting on them. As soon as they were removed, her attention began to focus on her single chick. I think it also helped that I moved the pair to another, larger nest on the floor of the coop. She was fine with that, too.
The two unhatched eggs were not viable, by the way. One appeared to be unfertilized while the other had a partially developed embryo. I have no doubt she would have continued to sit on the eggs if I had not removed them. As it is, she has been a doting mama to her chick, who is now eight weeks old and still being cared for.