Federer may make it to number 3, but it won't be for a while, and it will depend a lot on how Ferrero does, as much as it will on himself..
Federer stand a good chance of being passed by Moya at the next two masters series. Ferrero defends next to nothing in Indian Wells and Miamia, as does Moya. Federer defends a final, and a third round. So unless he manages to win one, and go really far in the other AND Ferrero loses early in both, he's won't pass him.
In fact, unless I'm not considering something here, Federer has minimal chances of passing Ferrero untill Monte Carlo, but keep in mind that Federer also won Hamburg, so those two pretty much cancel each other out. The real "contest" for number 3 likely won't come untill Roland Garros, where Ferrero could repeat his finals feat and still lose it..