Are all of the chicks the same color? Were the killed chicks the same color as those she is caring for? Did all of the chicks hatch at the same time, or was it a staggered hatch? If a staggered hatch, she imprinted on the first chicks and all others are regarded as intruders. Staggered...
Then there is no question that the mother scalped this chick and killed the other two. I would take the remaining chicks from her and never use her as a broody mom again. Sorry that you have to deal with this.