bases loaded groud ball

Question
We had an interesting situation occur in our game last night.  We were in the field and the bases were loaded.  The batter hit a ground ball to third.  The runner on third didn't move.  Our third baseman tried to tag the runner but he never left the base.  He then stepped on third.  The force out was called.  The question is when is the runner required to leave the base?  Was this the right call?

Answer
Hi Kris,

Well it was the right call by the umpire because F5 didn't play it properly.

A force is when a runner loses the right to a base they are occupying because the batter becomes a br.  R1 in this case can no longer legally occupy 3rd.  Had F5 simply tagged R1 first no matter where he was and then stepped on the bag you would have had a double play.  By not tagging R1 and simply stepping on 3rd the force is now removed and R1 can again legally occupy 3rd.

Think of it this way...same situation only F5 bobbles the ball, R2 advances to 3rd and R1 is also standing on 3rd.  If you tag R1 on the base is he out?, of course, because R2 is legally entitled to the base and R1 is not.  

Mark