3rd base

Question
How far off third base does a 3rd baseman need to be?
Also, if the ball is hit between shortstop and third base, whose play is it?
My son plays third and the coach has him about 3 foot off the base and when he hits grounders about 10 feet from him, he gets all over my son for missing the ball.  I just want an explanation of how this base should be played.  Thank you!

Answer
well this depends on who is pitching.If you have a guy who throws very hard and you don't think anyone will pull the ball down the line then he should be far off the line.If it is the opposite then he should be playing closer.Every coach is different but I would rather my 3rd baseman play for the ball in the whole as opposed to the one down the line unless it is late in the game and your trying not to give a double up down the line.