What better way to start off the new year than with the history of a number?
Shut up. It’s fun.
As a word, three comes from the Old English Þreo/Þri/Þrie and since Þ is just “th”, that makes them threo, thri, and thrie. Why so many? Because of course we had to have a feminine version and two different masculine versions. Anyway, all those versions of three come from the Proto Germanic thrijiz, and even earlier the Proto Indo European trei-. So this word really hasn’t changed that much.
The symbol of course has its own story. All the numerals we use in the English language originated in India before Arabic countries picked them up and then spread them through the European countries that we get our language from. 3 itself has changed over the centuries, sometimes jagged, other times written on its side with a huge tail. Basically, there were a lot of different versions of 3 over the years, and for some reason one stuck around when the others didn’t. Because that’s always how it is.
Tony Jebson’s page on the Origins of Old English