What does it meant to have a tri-malleolar injury to the ankle?

An unstable break. The inside bump and back of the tibia are broken, and the end of the fibula (the outside bump on your ankle).
3 points of fracture. That in your ankle fracture, the inner and back of your tibia, as well as the outer bone (fibula) have been fractured. It is common for the fracture of the back of the tibia to somewhat dislocated. Surgical reduction with internal fixation is the standard treatment.