Hi, Katie,
the rule of thumb - and there is some wiggle room here if you are paying close attention to the ducklings - is 90F the first week, dropping about 5 degrees a week until the outdoor temperature goes no lower than their optimal temperature, until they are fully feathered and acclimated to outdoor living.
Ducklings surprise some people with their behaviors especially with water and feed. If you find the way to manage their water, many concerns become less of an issue until the time is right for them to be outdoors.
Some people put ducklings outdoors at just a few weeks old in chilly climates. And many times, the ducklings survive. It all depends on what kind of shelter they have, their diet, the number of ducks, the breed/size, and a few other things.
So at three weeks, the minimum recommended temperature for ducklings is 90 - 15 = 75. In that case, I would say the night temperatures there are too low. You could provide a heat source, and some things I strongly suggest are (if you go that route), get a thermometer out there with the ducklings, be absolutely sure there is no fire hazard with your heat source, no risk of burning feathers or skin, no drafts, no way for weasels or raccoons or other predators to get at the ducklings, and you still need to manage their water so that you don't move a wet, messy environment outside where it can be a risk to their health.
A number of us have an area in the ducks' night shelter where they can have food and water, but it stays out of the sleeping area. Some people keep food and water (no food without water - ever) away from the ducks at night. This is not a good idea if the ducks are less than about eight weeks old.
Another option is to have a safe secure outdoor area where they can play during the day, and come into the brooder at night. They need shelter, water, and food outdoors, but with the temperatures above 75, they would likely be okay out there and then their use of the indoor brooder would be reduced quite a bit.
Do you have a thermometer in the brooder, by the way?