I'm not sure about the answer to your question if YOU owned BOTH horse 3 and 5, as they'd both finish at the same time due to the movement bar. (The prize money would be the same in either order, in this case, but the finishing order would affect the odds of any wagers placed on the horses.)In this situation it will be the owner's choice. From the rules on page 8 (last page).
If several horses that a player owns finish at the same time (after the horse on the horse die has finished), the player may choose which horses finish, and in what order.After the horse die is resolved the player also has the option of not moving any other horse that they own.