All three-horse trailers have a maximum width of 102 inches and lengths of 12 to 29.5 feet, according to The Chronicle of the Horse. The length depends on the trailer style, with stock trailers being the shortest, and whether the trailer includes a tack or dressing room.
Three-horse trailers are designed with gooseneck rather than tag-along hitches to accommodate the weight of the three horses. Horses are loaded on a slant, with a minimum of 4 feet allotted for each horse. Three-horse trailers shorter than 20 feet may not accommodate larger horses, such as thoroughbreds or draft horses, warns Equisearch.com.