
Alexander K. answered 03/14/19
Recent College Graduate
Hi Shania N.,
This is a great question with a very awesome answer! As you know, English is an old language and has developed significantly over many centuries. In medieval manuscripts, we can see a symbol or marking that is often called a "subpuncting" or an "underdotting." The main use of this mark is to indicate when a word has been taken out of the sentence. The three dots are placed under the word that is to be omitted. In this image you can see a word that was written by mistake and instead of putting a strike-through mark, has opted to blot the word out and at the three dots underneath, hence "underdotting"
I know this doesn't entirely answer your question since you'd like to know why there are specifically three dots, however some scholars believe that early writers just starting using three to make it not look like a mistake (of just putting one or two as they were using ink at the time which tends to drip). This method could have caught on and become widely used by other writers at the time.
Link to Image
https://compote.slate.com/images/dcc8b54b-cbd9-4e07-bb70-74ff194308c6.jpg