I broke my right collarbone 3 months ago. Should I remove the plate or not, and if yes, when.?

Plate is unusual. The clavicle is the most common fractured bone in the body. Vast majority of time it requires a sling, immobilizing shoulder, pain control, then some physical therapy. If a plate is needed, probably needs to be there at least six months, but that is orthopedic (which I am not) assessment who did the surgery.
Not necessary. Generally, when a plate or rod is inserted for fracture healing, it does not need to be removed unless it is causing a problem, such as protruding, pain, loosening, etc. Best to consult with your orthopedic surgeon, preferably the one who did the original surgery.