In Annie Proulx's short story, the phrase "three-up outfit" appears, used to describe the ranch of one of the characters. I do not know what "three-up" means.
three-up outfit A small ranch which, as one cowboy said, "Don't own 'nough beef to hold a barbecue."
— Western Words: A dictionary of the Range, Cow camp and Trail by RAMON F. ADAMS
Pub. 1945 by University of Oklahoma Press.
That same dictionary also has:
top screw — The foreman.
so presumably screw = cowboy. And it defines :
three-up screw — Said of a cowboy working on a small ranch where three horses are considered enough for a mount.
mount — The number of horses assigned to a rider for his personal use during his stay at the ranch...The number of horses assigned a rider depends largely upon the size of the ranch and the kind of country to be worked. Seven to ten head is an average mount..
so extrapolating I think we can say three-up outfit could be regarded as a small ranch where each cowboy/rider only needs three or so horses.
I think the three-up is unrelated to the number of cowboys and indeed from snippet below, it seems the hero is running the ranch alone, initially at least.
For reference, below is quote in context from Close Range: Wyoming Stories - Page 98: