Some folks can sort of rig the thermometers up, maybe we'll get some pics here soon.
By 3 wks, I think some time in 70'F temps should be ok, I guess I'd be concerned about the overnight temperatures. Remember that ducks are flock animals, so in a flock of 8, they're stronger as a group than...
How old are these ducklings? 90'F the 1st week, 85'F the 2nd, 80'F the 3rd.... And so on. A thermometer is your best tool. Do you have one, and if not, can you get one?