How to index 3-letter words?

According to MySQL documentation on Fine-Tuning MySQL Full-Text Search:

" 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. "

To rebuild indexes of tables used by YACS you can visit the Control Panel, and select the Optimization link.