Why is it if the giants win the next three games and dallas looses only one of there three games and there tie. How can the giants be the division winners?

Giants beat them in Dallas by more than Dallas beat them at the Meadowlands. Also Giants have a better divisional record. If they beat Philly and Washington, then their only divisional loss would be to Dallas.

If you go to NFL.com you can find the various tiebreakers.