Just thought I'd throw this out there for any young players. If you have a weak third finger and want a different way to sort it, try moving your fingers forward one valve (so that your second finger is on your first valve) and practice exercises like Herbert L Clarke Study 2 in the keys of F, G an A. It's just for variety and brain training. Is sometimes helps.
PS Can't wait for Rowuk's reply. I think he'll shoot me down in flames.