Miss Lydia gave some good info (as always). The only thing i would say about the weight thing is - are you certain that all of your ducklings came from the same hatch?
I've gotten ducklings before that I was told were 2 weeks old. I had one two week old duckling at home that I hatched from an egg so I KNEW the hatch date on that. Of the 4 that I got, 2 feathered out much earlier (10-14 days earlier) than my known 2 week old duck. One was about the same as that one and one feathered out much later (about 7 days later).
Basically what I'm saying is - when some people say "3 weeks old" that means "the first (or last) batch hatched out three weeks ago, I've had two other batches hatch out since then and I keep them all together". So weight might not work in that case.