Part of Top Gear's appeal, to me anyway, is the quality of production and the way it is shot. Even if the three of them make another show on another channel, I doubt it would have the same style and production values. You only need look at some of the shite "entertainment" shows that Sky or ITV make. Look at stuff like Braniac, good idea for a show but so poorly made and very remnant of the "Planet's Funniest Animals" type of American tripe. And the less said of that new Sky produced game show, "Wild Things" the better.
There's a lot more to the Top Gear magic than just the presenters that I feel a lot of people are forgetting.