My runners are pretty small, they are tall and slender. To me they are small, compared to my Pekin and my Muscovies. I am sure if you put three in there and it is just for the night they would be fine.
And my runner is a good layer too. I think she's pretty, she's fawn and white, but they...
Be sure to figure out something for ventilation. That might get pretty hot in the summer with the birds in it when it is all locked up.
Runners are great birds, I love mine.