One of the most frequent questions I get asked is why I do total body workouts with my clients (and for myself). There are several reasons behind why a total body workout 3 times per week is better than body part split 3 times per week, or even 5 times per week. For 48 hours after a resistance training workout, your muscle-protein synthesis is elevated. By doing another total body workout 48 hours after the first one, muscle-protein synthesis levels remain continuously elevated, allowing your muscles to strengthen and grow continuously. Hit the resistance training too soon after your last session, continuously, and you'll be pushing the muscle-protein synthesis to elevate before you've had adequate time to recover, leading to under-performance in subsequent workout, leading to less or no results.
Also, by resistance training total body 3 times per week, each muscle group gets worked 3 times per week. If you do chest on Monday, legs on Tuesday, etc, each muscle group only gets worked once per week. After 96 hours of no stimulus to that muscle group, the law of reversibility kicks in reversing the effects of training. This would happen every week, for every muscle group in the traditional body part split.
Here's an article from Men's Health outlining this very question. Alwyn Cosgrove, owner of Results Fitness, one of the most successful gyms in the country, is featured.