Rendon injured himself rounding third base this time around. He injured his ankle previously in a rundown as the runner. His shoulder muscle strain that bothered him his junior season would certainly not be better off at 3B than at 2B.
He may be too fragile for baseball, but I can't see any evidence that if he's NOT too fragile for baseball, then any position might be more likely than another to keep him healthy.
To actually attempt a mindfact on my own, it seems to me that more 3B are just as, if not more subject to sporadic injuries than 2B, just due to the nature of the position. For example, last year Longoria, Beltre, Youkilis, Polanco, Chipper, and Zimmerman all missed significant time.