If you read the article introducing the kittens concept, you'll see that the author intends it to be customized to each site, thus preventing spambots from simply memorizing the pictures. And randomly picking three out of 9 images only gives a possiblity of success of 1/84, better than many word captchas are achieving these days.

Anyone who wants to custom-program a bot for a single site would just be better off manually posting their spam.