force out/runner tag out

Question
Bases loaded, one out...
Ground ball is hit to third baseman who fields the ball at third base before runner leaves third base...
Third baseman tags runner (who is still standing on third base), then steps on third base (forcing out the runner approaching from second to third)alleging that he has executed a double play.

Please, please correct these brothers. This is what Gary Sheffield should be concerned with. At least six men over 40 have insisted that I am wrong in declaring that the tag the third baseman applied meant nothing because the runner was on the base (alone) and that the defender removed the force situation from the runner on third once he stepped on the bag. Thus only one out was made and the bases remained loaded with the batter on first, the runner who was originally on first in now on second, and the runner on third can remains there because of the incorrect execution on the fielder's part.

Answer
I believe you are correct.If the runner is on his occupied base and was tagged he cannot be considered out.The fielder should have tagged the base then went to first or went home but the runner is safe unless he would have been tagged while the runner from second was standing on third base.