What are three letter words?

Three letter Words are simply Words containing three letters.

If you are asking for a list of all three letter Words, Webster's Second International contains 1 ,351 of them - too many to list here!

If you have access to a non-Windows computer, copy/paste this into the command line:

grep '^...$' /usr/share/dict/Words

If you have a Windows machine, I believe you can download grep for Windows, then find a free plaintext dictionary and have a go at it yourself. There are also text apps that can search with regular expressions, like BBEdit.