This always comes up...
There is absolutely nothin wrong with training a muscle group more than once per week... As long as the volume is adjusted accordingly.
If you simply took the chest portion of a typical "once-per-week" split routine and performed that three times a week then yes you would more than likely quickly overtrain. The difference is that those of us who do train each muscle group more frequently than once per week don't obliterate our CNS's with set after set of pointless isolation exercises. Instead we stick to the compound lifts and up the frequency.
Basically the fatigue is accumulated over the week instead. Take Bill Starr's 5x5 for example. This routine has put on more lean mass on atheltes than any other over the last 30 years. On it you squat, bench and row three times per week and do very little isolation work. The volume for each muscle group is far less per day but the fatigue is accumulated over the week. Rather than blasting a muscle once per week we hit it three times, but with less volume per day. Most people respond extremely well to this type of training.
Having said all this I disagree that splits are nonsense. They have their place and some people do respond fantastically to them. I personally think they are fantastic for beginners (though that is just a matter of opinion) as they are easy to fit in around peoples schedules and sort of "ween" the trainee in to training. For pure mass gains though nothing beats something along the lines of Bill Starr's 5x5 or DFHT.