What if a runner rounds third then retreats only to find a runner from second is now on third and he retreats to second but is tagged out what happens to the runner on third?

Lead runner has the right to the bag as long as they are not being forced ahead by occupants of other bases. If the runner on 3rd was not being forced they can return to the bag and the runner from 2nd must return to 2nd. The runner returning to second if tagged is out and unless the runner returning to 3rd is tagged while off the base then they are safe. If both runner attempt to occupy the base at the same time and fielder tags both runners while they occupy the base the runner from 2nd is out unless they are being forced ahead then the runner from 3rd is out.