Make a list of all three-letter sequences. Divide them into two groups - those which you could imagine someone writing a clue for in a crossword puzzle, and those which you can't think of any way to get them into a crossword puzzle. DDT is obvious, DXD not so much. Perhaps use one of those acronym finder services to locate unused acronyms.
Then, using letter frequency ranking on this list, find the one that can't fit which is closest to fitting. This would be based on letter frequency and position. i.e., a 3-letter acronym which has the highest letter combination + letter frequency by position scores among these non-existing TLAs.
Then, make a movie and use that as the title!
After your movie goes on DVD, whenever crossword puzzle makers need that 3 letter combination, your movie will be the only possible clue for them to use. Over the years, your movie will enter popular consciousness. I predict your sales could rise by as much as 5% from this tactic.