I am with Momma on this one. Here is my reasoning.
Ducklings can "handle" alot of things. Colder temperatures, inferior nutrition, inadequate water, exposure to disease . . . sometimes. Maybe even more than half the time. But not without some effect. It depends on the duck, and the frequency of less than ideal conditions.
At worst, they will sicken and die. At best, their development may be delayed or impaired. We cannot see much of what their little bodies are doing, because it is organ, skeleton and neural development.
Three week old ducklings need to be at around 75F, give or take a few degrees. I cannot trust my "hand" thermometer because I found, by experimenting, that the air temperature really affects my perception of warm and cold. When it's 30F out, like today, 50F water feels nice and warm. When it's 60, the water feels cool, and you get it, the warmer the air the colder the water feels. Also, I am used to New England weather, so 60F is balmy, and for my relatives in Tampa, it's bone-chilling cold.
So the short response is, I don't think you have committed a crime, we (I) can get very overprotective. But I still have nine of my eleven runners, and the docs say we could not have saved the two who died - one from egg yolk peritonitis, and the other from a not-sure, but probably peritonitis.
I would go with warmer water based on a thermometer reading. You did great by drying them off. What temperature is the brooder?