MySQL full text search for words with three or less letters
This section of the manual might interest you : 11.8.6. Fine-Tuning MySQL Full-Text Search (quoting a portion of it) :
The minimum and maximum lengths of words to be indexed are defined by the
ft_max_word_lensystem variables. The default minimum value is four characters; the default maximum is version dependent. If you change either value, you must rebuild your FULLTEXT indexes.
For example, if you want three-character words to be searchable, you can set the
ft_min_word_lenvariable by putting the following lines in an option file:
Then you must restart the server and rebuild your FULLTEXT indexes.
(You should read that page, for more informations I didn't copy-paste ;-) )