The thigh bone (the femur) has two distinct 'knuckles' at its lower end so rather than having one weight-bearing surface, it has two. The top of the shin bone (the tibia) is divided into two areas to match the femur. The main weight-bearing interaction between these 2 bones is therefore shared between the inside (medial) and outside (lateral) surfaces.
OK - that takes care of 2 compartments, but the third.......
This is where the kneecap comes in. The patella interacts with the femur. There is a groove in the lower end of the femur that the patella glides up and down in as the knee bends. The patella does not directly articulate with the tibia.
When your knee is worn it can be isolated to one area or involve two or three.
Having tri-compartmental arthritis does not necessarily mean it is worse than uni-compartmental as it describes the location of the damage, not the severity.
It may however have a say in what treatments are suitable for you