So many people die everyday. I think it's so easy to turn a sequence of deaths into your "3". I find the whole thing silly, it's just an old superstition that people can't seem to let go of.
I think if you expect there to be 2 more deaths this week then if someone you know (or know of) does die then that will jump out as number 2 or 3. It never occured to me to count deaths in my life, but I am pretty sure they never came in 3's. It was an individual each time, didn't feel the need to wait around anticipating that third death.