Not to mention all the one- and two-letter combinations!
Well there’s about 1/26th as many of those.
There are some more three-character user names if you count dashes and digits, which are also allowed in GitHub usernames. Although I think the first one has to be a letter of the alphabet.
I have a file by each of those names inwhich each bit represents one article that either has the string or not (1 or 0)