arabianequine, two of the first things I look at with feed are protein and niacin.
Ducklings need three times the niacin that chicks do, so if you use chick feed, you are very likely to have insufficient niacin, which leads to serious neurological problems (Holderread). It is easy to take care of - add niacin to their water (be careful, too much of a good thing is bad, too).
I cannot remember the numbers off the top of my head, but do a search on BYC for my post about how much to put in their water. Some people sprinkle brewer's yeast but again, I don't know how much. A search will probably turn up that information, also.
Ducklings, according to Holderread in Storey's Guide to Raising Ducks, do well with 20% protein the first two weeks, 18% the next two, then about 16% until they start laying, when it goes up (I think . . . ) to about 18%, then back down again when they are not laying.
The book is worth having - it describes much of the details of good nutrition and feeding.
By three weeks I am thinking you want to begin to switch to grower feed, though anything is better than nothing.
With my small flock, I just bought rolled oats (not instant, not steel cut) at the grocery store. At three weeks you can give it finely shredded greens (not so much spinach, which interferes with calcium absorption, I am told). I have given dandelion greens, cress, chard, baby green mix, and red lettuce.
I also give them just a few little fresh green peas (I thaw out the frozen kind). Canned peas are not a substitute - think fresh or at least fresh frozen and thawed. Watch to make sure they don't choke, and it is best for them to have a little sprinkle of grit on their ration, to get their digestive systems geared up.