I see photos of braids in the mane and forelock of roadster horses. My thought is you just braid a part of the hair as the third strand but what do you do when you run out of hair?
Although you technically can braid however you like, there is a traditional way. Two ribbon of one color and an accent of another color. All three strands have hair folded into them, so when you run out of hair you can just keep braiding down. The top and bottom of the ribbon, fold in half and cut at an angle so it makes two points.