Are all three-letter acronyms in use?

Try the following -  QAJ   QAY  QAZ

I don't know for sure, but I think that we can safely assume that there are  three-letter acronyms that aren't used. Let's check the amount of possible three-letter acronyms. Since the alphabet has 26 letters, the number of combinations is 26 to the 3rd power, or 17576. That's quite a lot.  I'm sure that even without empirical research, we can assume that there are quite a few combinations without any "acronymic" meaning.

Nevertheless, I did some (very) minor-scale "empirical research": In order to find examples of unused acronyms, you shouldn't check combinations with very common letters, but go to the more rare ones. For instance, The following combinations weren't recognised by the web-site you referred to in your question: qqq, xzw, xey, uze & bjf. Those are mere examples. I'm sure that you can find many more. 

I`m shure you can find 3 letter accronyms, try to make combinations with  the letters q w z y, i`m shure you can find what u need, 

You can also find comprehensive explanations for SEX acronyms or GOD abbreviations using OXID Acronyms and Abbreviations located at:

http://abbreviations.oxid.ro/