^ ditto ^
I have 3, and my run-in is 12x30-ish (36 minus the part that's fenced off for the water hydrant/electical) It's plenty wide enough, and all three hang out in there together. I wish it were deeper though because the snow will blow in all the way to the back wall sometimes. It's facing the "right" direction to avoid it most of the time, but sometimes well... the weather doesn't behave.
I have three geldings who live out together. Their shelter is 12x24, with a center divider (not solid). Typically, two always hang out in one "stall" together, while the other one lounges on his own. I've found this configuration to eliminate someone getting blocked out into the weather.
We make our sheds 12' by 27', because 12' is the widest we can haul down highways if and when we want to move them and 27' fits out material best.
Our sheds are portable, we just tie them down on each corner with chains attached to the corners, with an added large bolt at the bottom horizontally and dropped in a hole we fill with 2-3 sacks of concrete and have not had one blown away yet.
Right now, we have two put together, so we didn't make but two walls for each and have a panel in the middle, where they come together.
We have had up to five horses live peacefully there, but they got along well.
With the panel in the middle, if one is not welcome on one side, he can still stand there on the other side without being run off on principle.
The way our storms hit, when we have blizzards, unless there is an overhang in front, the wrap around snows do blow far into there, here mostly on the West end.
We could leave the East end open and no snow or rain would get under there.
You may want to experiment with that and change according to what you see happening.
We use a 12 x 24 with an overhang for 3-4 horses. An overhang is a good way to add "size" to the shelter without adding as much cost. Also, if you have a "submissive" who is not always allowed to share, it lets them get shade without infringing on the alphas space
The red bar was to keep everything straight there and while dragging it out to the pens and was cut out once in place.
For horses, you want to line the bottom 4' with 3/4" OSB or outside rated plywood, as a kickboard.
Those may have been some we made 12' by 30', to fit better in some cattle pens.
For cattle, you want 5 square feet per animal and that length was better for that purpose.
Most any place you buy metal can tell you who is a good local welder that will make such for you, or any design you may choose, with overhangs of all kinds, extra partitions and the kind of framing, slope to the roof and material to fit your weather.
On my dad's boarding farm we do 2 acre paddocks with run-ins and rarely use the stalls. 12 by 24 is great for 2 horses, not 3. 3 are OK if, and only if, they all decide to get along perfectly for the day. Any little tiff and one is out in the rain.
I do feed hay in them, if that makes a difference. There are more disagreements and i have to clean them, but it reduces hay waste significantly.
I have 3 and am building a 14 by 36 run-in, scheduled to go up next week. We have found this size to be wonderful for 3 horses, which is what I have in my largest field, everyone has plenty of space to get in and away from each other. I like 14 deep because of snow, you might not need that in NC.
The deeper we make a shed, the more height it needs, so air and sun get close to the back, so it is not a dark, dank mess back there.
Our permanenet working cattle shed was 20' deep and we went considerably higher to bring air to the back of it.
When protecting in front of a shed, the more height, the further it will protect the area in front of it, so even if horses are not directly under the shed, they still have protection from blizzard winds standing in front of it.
The best sheds are those animals have access to it all around, so they can stand on the side they like best.
For management, some times we prefer sheds butt against a fence on the back or sides, or is part of two pens, each with half of the shed, as in these roping steer pens.
Those are two 12'x27' sheds, that we built 5 years ago:
We are just finishing putting up a 24x48 shed for 6 horses. They all get along good and I think it should work out fine but for 3 horses I think you need a min of 12x24 and even then one may get kicked out depending on how your herd gets along.
I have three horses and built a 12'x36', with dividers every 12' to prevent one (and you know who you are, Fling!) from hogging the whole thing. It's really 16x36, though, since it has a 4' overhang. I made a large overhang because we get (or usually get - when we're not in the middle of a 150-year drought) 50" of rain a year. I wanted a large overhang to ensure water did not get blown in. We've had it about 6 years and has worked great. No turf wars and everyone who wants in can get in.
Mine is 10 feet deep by 24 feet wide. I was always told that making it wide was more important than making it deep. If you do the opposite and it's not wide the dominate horse can stand in front and block it.
Mine is 12 X 28 with a divider in the middle for 3 horses. I have gates on front that are open. I don't have a barn and this allows me to quickly make an emergency stall out of half the shed while still having space for the other two in the other half. Works pretty well. All three are using it this summer but one will sometimes stand out in the winter. Go figure.
Phaxxton, the shell of my 14 by 36 is up and I love love love the size for 3. When I get the metal on it next Tuesday I will post pics.
If you want pics of the shell before then to get a sense of the scope, let me know and I'll post a few.
I originally wanted it 10' high short side with a 3/12 pitch but that ended up being TOO high, it just looked weird. I had them lower it to 9 ft. and it looks perfect, easy to drive a tractor in if needed though I don't do that, I feed squares.