Yeah, that drives me nuts, too. Unfortunately, it's not a restriction I've put into place, and not a restriction I can bypass.
If you want to know the details, the issue is MySQL, the software that runs the database that stores all of our data. It's default minimum character requirement for a fulltext search (the type of search we use) is four. It can be changed to three or lower, but it causes a large hit in performance. Our host, like most other hosts, keeps the limit at four because lowering the limit on a busy shared server will likely bring the server quickly to its knees.
I do have an alternative, though. Google spiders the heck out of our site, and so you can use Google to find posts. All you have to do is add this to any Google search:
So, here are a few examples of searches for three letter words: CIA
I do plan on integrating a custom Google search into the site as an alternative means to search, but it is not a high priority at the moment.
I hope that helps a bit!