They Go In Threes?

I've always heard that saying: When someone dies, they always go in threes. Now, I'm not exactly sure how that works. Is it three in your family? Three that you know? Three within what time period? And does it stop with three, or does #2 of three have his/her own three? 

Depending on the factors, either the threes have already happened, and exceeded, or they have just begun. Within this year alone my distant family lost Mary Alice, Edna Ruth, and now Ted. Then there was Pa Claude, Horace, and Carolyn. And it's just the last day of February. 

Horace and Carolyn were not family, so do they count in the equation? And Pa technically is not blood relative, so does he count? Either way, that is two sets of three. So do we start over now, and if so how long do we wait? 

Even in Hollywood they come in threes. Probably more so than everyday life. Tim Wilson, Shirley Temple, and that dude that died of an overdose. I am sure there are more we don't know about or hear about. 

Where did the saying come from, that they go in threes? And why do you suppose they do, when they do? 

Deep thoughts over a 2am, 1 1/2 oz drunk bottle. Yes, I am sleep deprived, and parts of my body are going numb. Need sleep!