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_min_word_len and ft_max_word_len system 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_len variable by putting the following lines in an option file:

[mysqld]
ft_min_word_len=3

Then you must restart the server and rebuild your FULLTEXT indexes.

(You should read that page, for more informations I didn't copy-paste ;-) )