Hmmmm.......I think that the best way to do it would be have Harold beat William first, and have to march north. As it was mentioned, Harold was the much better general, commander, etc.
If he beats William, the fyrd will still likely have to go pull in the harvest, leaving Harold with fewer men, and a good portion of his troops are gone.
Harold marches north to deal with Harald and Tostig. Harold must almost assuredly die, as it is unlikely that the Saxons will lose unless there is some great low to morale. So Harold dies, Harald and Tostig win.
I think these Vikings will have the same problem the others did. England is just too far away, with too many pesky nobleman. The Vikings and Tostig might control the old Danegeld, but eventually, somebody, likely a Saxon king, is going to conglomorate all the little princedoms a la Alfred the Great and Edmund Ironside. Basically a repeat of the 800s.