This three mule team belongs to our cousin who is also the driver. The mules live in the pasture next to our home here in Southeast Missouri. Our cousin lives on the same property.
This image shows in good detail the mules and their harness which allow them to be connected together to the breaking plow and to pull it together as a team. The collar around their neck is what carries most of the pulling load. The harness connects to the collar then goes down the back and around the rump of the mules. This harness contains the chains or straps that connect the mules to the plow and keep the chains from rubbing on the mule. These chains or straps are connected to what is called a “triple tree”, one “single tree” for each mule then each “single tree” is connected on one long rod that is in turn connected to the plow. This allows each animal to pull an equal share of the load. I think they are called trees because back in the old time they were simply made using a limb from a tree.
You can also note that each of the mules has a rope which goes from halter to halter between the three so they can not move away from each other. The two mules in the center and on the left have a rod connecting to the bottom of their collars. this suspends the tongue of the plow and keeps them from moving to close to each other. They also have "blinders on their halters so they do not see things that distracts them and makes them shy suddenly away from their team members. The driver has two reins, one on each outside mule. When the rein is pulled on either side it makes that mule turn and the other mules have to follow because they are all connected at their halters.
It is all a very ingenious arrangement and was developed by the farmers of olden times over many years of learning how to use draft animals to pull their equipment.

Nikon D40x with Nikon 300mm VR lens on auto and shooting RAW