I'm pretty sure that would be scored a base hit. At worst it would be ruled a sac-bunt fielders choice.
How do you give the 3B an error on a play that 9 out of 10 smart 3rd basemen will make most of the time if they think the ball is going to roll foul?
I'm not sure that I agree with the bold above. The OP states that making the play would have resulted in the runner being thrown out by "15 feet". Third base was my position growing up. If I knew that I could throw out the hitter, I made the play. If I thought the hitter may beat out my throw, I would allow the ball to roll and hope it rolled foul. The runner on second would advance to third, but I would still prefer to get the out. NOTE: There are circumstances when this may change, but I am basing my decision strictly on the information provided.