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.
Joining the crowd, food coloring is visible, not toxic and won't constrict/etc as they grow. As it fades just re-apply it.
And we need pictures and/or video of the new little ones.
You can go online to various hatcheries, check the state thread on this site, etc. Massachusetts is pretty far north, so figure it's probably still cold. May is only six weeks away. Why not just wait until May when the weather will likely be warmer?
A couple of thoughts:
Dogs and ducks can co-exist, but you'll need to be careful. Dogs descend from wolves, so they have predator instincts. Ducks are prey animals and they'll have prey animal instincts. A dog can trigger a duck's prey instincts, which in turn can trigger the dog's predator...
In that case, you should contact your state's Department of Agriculture and see if you can get a free or low-cost necropsy on the dead ducklings. If you know what's killing them you can possibly take steps against it.