Alexis Moinet wrote :
yes you must add ft_min_word_len parameter in your mysql config file and then rebuild the search index
see http://www.mediawiki.org/wiki/Manual_talk:Configuration_settings#using_searc...
(NB : I think ft_min_word_len=3 is enough for 3 letters search, use ft_min_word_len=2 only if you also have two-letters words)
Also, note that there is a list of stop word in mysql (see http://dev.mysql.com/doc/refman/5.0/en/fulltext-stopwords.html), i.e. words that are not indexed (the, and, ...) so if you have three-letters acronyms that match one of these stop words, they won't be indexed anyway.
if you want to remove/change the stop word list, add this in the mysql config file (in the same section than ft_min_word_len) :
ft_stopword_file =
or
ft_stopword_file = filename
(PS : don't forget to restart your mysql server after a change in config file)