The unofficial rule of thumb is 4 sq. feet of indoor space per bird for standard breeds, 2 sq. feet for bantum breeds. Especially if you live in a cold weather area, you want as much indoor space as you can get in case of bad weather that the chickens won't go out in. I don't think having three chickens would be a problem. There's always going to be a pecking order, whether you have 3 or 33. I think three will do fine together. With low numbers, especially if you have friendly breed, there's a strong chance for harmony. I would be more concerned about trying to fit four birds in there. Crowding is more likely to result in pecking problems than an odd number of chickens.... Have fun with your new birds!
ETA...folks have nesting boxes of all sizes. I would worry a bit about your nest box being too narrow...maybe not enough turn-around room?? For three birds, one nest box would be fine.