Generate Random ID that don't already exist

I have a table containing IDs and they are all in 3-letter words. How do I use SELECT statement that returns a randomly generated 3 letter ID that does not yet exist in the current table and ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789 are the only characters that can be used?

I would like to provide something I have in mind but I totally have no idea how to do this. Thank you so much.