I've only seen two of Jo's pre-Three Doctors stories myself: The Claws of Axos and The Sea Devils. I couldn't stand her in the former, but revised my opinion when I saw the latter (which comes exactly one season later). So I guess maybe she just gets gradually better over her whole time on Who.

And I like your point about the rite-of-passage feel to Three's era. It would be unfair to say that he is immature as such at the beginning of this incarnation, since he comes across as already very strong and wise in The Silurians. But I agree that he is constantly having to battle to prove himself worthy throughout this period - explicitly to the Time Lords who won't let him off the Earth until he does, but implicitly also to himself and to some more nebulous sense of a higher Universal power.