Since you probably don't have the previous corpses, if it happens again you might want to contact your government's agricultural agency to see about getting a necropsy so you can find out what's going on.
You might think about separating her from the other two so they won't bully or trample her given the significant size differential. Maybe put a mirror in with her so she won't be as alone once she's separated.
Others will probably have a better idea of how to boost her growth, but it could...
Probably an adult - and probably a drake at that unless you're willing to get multiple females (three or more). Adult ducks have a tendency to bully or kill strange ducklings. Meanwhile, a single female can be overbred and killed by a horny male.
Raising and introducing a male duckling would...
Like TomCahalan noted, figure they don't knare (don't know and don't care). Still, do they not have a phone number? While mallards aren't protected in Australia, you want to just make sure you're not breaking some local municipal code/policy or something.
Honestly if you were going to train ducks to do anything it would be to use a litter box or some other designated "poop area" for easier cleanup. I recall an article by some animal behaviorist who said it was possible, but have yet to hear of any actual human beings accomplishing this miracle.
Do they have their adult feathers yet? No firsthand knowledge myself, but most people here and other places have said that as long as the temp doesn't go below 50 degrees Farenheit (11 degrees Celsius), they should be okay outside.