Well, the F3 we know about were in a huge lead for a large part of the race from at least Hong Kong and on (maybe even sooner). Maybe that helped ensure Jill & Thomas would stay with Nat & Kat and Brook & Claire.
I don't think the final 3
will get way far ahead, at least not way early. The producers have largely made such a thing impossible with the way the flights are arranged. I suppose it
could happen if the 4th place team decided to quit on the non-elimination leg, but why would you quit if you're in the Final 4?
I think that the fourth place team
was with the other three, but somehow we missed a team in our sightings. They wouldn't have to be days behind for such a thing to happen: remember how originally we had no sightings of Kisha & Jen beyond, what was it...Leg 2? But despite the "odds", Kisha & Jen somehow made it to 4th place. We could have another "missed" team scenario here - "last seen" doesn't necessarily mean that was the last time the team was in the race.
EDIT: Turns out it was confirmed that on Leg 11 there was a team hours behind. But we still don't have any real clues to the identity of our 4th place team.