I agree most of those won't be broken. I think the Indy/F1/Le Mans one is still pretty doable though. Juan Pablo Montoya won the 500 and finished 3rd in the F1 Championship twice in a row (he did however win at Monaco and Monaco, Indy and Le Mans are often considered to be the true definition of the "Triple Crown"). He never raced at Le Mans but he has won the very prestigious 24 Hours of Daytona. So basically, he hasn't done it and probably won't, but I think he's shown that it is at least doable in the modern era. A whole bunch of former F1 guys race at Le Mans every year and every once in a while you get an IndyCar guy like JPM or Sebastien Bourdais moving over to F1. All it takes is one.

As for the Jack Brabham one, I find it EXTREMELY unlikely, but I don't think its completely out of the realm of possibility. I was thinking that maybe a driver, later in his career, could buy the team (or at least a controlling stake in the team) he was driving for, and then win. Brawn bought Honda and won, so what would stop a driver from doing the same?