Help remembering a book

Hi all. I'm not even sure if this is a King book but I remember a story that had 3 men who not everyone could see, and they acted like the Greek fates. 2 worked together and would snip threads hanging above people's heads and they would die soon after and wore doctors clothing, and there was a third who would do the same except he was grimy and used a rusty razor. The protagonist could see them but no one else could. Help!