Recent history shows that playing 3 bigs just can't work outside of limited minutes in certain situations. Diaw and West can both post, pass and shoot but at the end of the day both can only hit consistently from about 18 feet. Teams will sag off one of those two and recover forcing them to put the ball on the floor or shoot a contested mid range jumper.
Utah tried this in '13 when they played Milsap, Jefferson and Favors together. They were very good defensively and crashed the glass but got murdered on offense.
Spurs will probably go Anderson-West-Diaw and have Boban play spot minutes or even start in the dozen or so games Duncan rests. At least with that unit Anderson has more range even though his 3pt shot is still a bit unproven. The Bench crew has been much more of an offensive unit ,anyways, for the past few seasons. As long as they can blow the doors off teams offensively and not be a joke on the other end they're be a plus.