Generate letter string from words in text field

I'm looking for a way to randomly choose three letters out of a single-line "organization" text field in my Contacts app to fill a "client code" in a related Client app. Seems pretty straightforward but, I also need to check all other client codes to make sure that it is generating a unique three letter combo. Ideally, the sequence of letters would be in order of appearance in the organization name. eg- Acme Company = ACM, ACC, ACO, etc. 

Has anyone done something related that they may be able to share?