Hi Woodcrafter, I've watched your post unanswered for a couple of days now so I'll take a crack at it. I'm no expert - but we have 4 Runners. I don't know how fenced in your nursury is or how many predators in your area, but sounds like you have some concern.

For night and cold weather, I should think you'll need a SECURE pen that predators can't crawl under or break into. Not sure what would be big enough for three ducks - but at min. I'd say 5'X 4' or equivalent? (My pamphlet says 4 sq ft per duck). Ducks in general are pretty cold resiliant, but I think dry is pretty important and free from too much draft. We put down straw bedding. It's a good insulator and they snuggle up in it and are fine. And oh, bear in mind that whatever shelter you provide, it will need to be cleaned out from time to time.

Not sure what a dog igloo is, but if it has a secure door on the opening and floor beneath then maybe you'll be alright.

Indian Runners are very patterened creatures we've learned. Once they get accustomed to a routine, they'll follow it like soldiers. If however, you move an item, or door, or the pen, or your schedule, they get all boloxed up in their little heads and act completely dazed and confused. Just an FYI, there.

Predators strike in the day too, so if they're completely free ranging, well...you've had good luck so far I guess.
Good luck and hope this helps. Google around for duck care and you'll find lots of advice or perhaps others will chime in too with more experience.

I bet your orphan is happy he has some friends to quack quack to.

LF RJ