selecting a mouthpiece is like finding the right pair of shoes... everyone has their own opinion of what feels right. if your current bach 3C is comfortable to play, you can hit all of the notes you need to hit, but you're looking for better tone, then you might want to experiment with different backbores to improve your tone. this is why modular mouthpieces with interchangeable cups/backbores exist. if your tone is good at low volume, but breaks at high volume, you might need a larger backbore. if you have trouble hitting high notes, a smaller cup might help.
i'm a low brass player who plays trombone most of the time, so that's what my embouchure is built on. i use a bach 3, very large backbore for a tenor trombone or corps baritone, most people play a bach 12C or a bach 6-1/2AL.. when i play soprano/trumpet, i use a bach 1B. for contra/tuba, i use a bach 18. i never could find a decent mellophone mouthpiece that worked for me..