I think you’re fine. When I’ve put chicks outside in colder weather, there have been times I would still bring them back inside at night. When I couldn’t catch them easily anymore, that has always been the end of that.
I would watch for interactions with the young duck to make sure that introduction goes okay, but with all such young animals, and only three involved, hopefully it will be fine. I don’t think the temperature will be an issue, except ... can the box be closed? Sometimes I find chickens hesitate to enter a dark, enclosed space (even my 10’x10’ walk in coop) if they cannot see inside. Since it’s also the duck’s territory already, they may not want to enter and try to roost on top or elsewhere. You may need to put them in and shut them in for a few nights, if that’s possible. Or you could wait and choose some slightly milder weather to put them outdoors. Mine go outdoors at 7-10 days and lose access to heat around five weeks.
