Question about Dev Wounds overflow

Hello, I have a question about how Dev Wounds convert damage. Let's say i have an attack go through as dev and is dmg 3. It goes in to a unit of 1 wound guard. Does that 1 instance of 3 dev wound only kill 1 unit or will it overflow and kill 3?

I was told recently that dev wounds were different from mortals and one attack no matter damage still can only kill 1 model. Is that true?