No such reg in BC
the only stipulation is for 'shot size of #1 Buck or larger', so tri-ball woul dbe fine
which brings up another BC rule, 20 ga can be used with proper size buck, but it is impossible to find, largest i have found is #4 buck
even for coyote, but #4 buck has been proven effective for them.
Maybe not much demand for 20ga deer loads?
You may get away with a Tri Ball load in most of the Province, but certainly not in the "Shotgun with Shot" Areas as "Shot" is defined on Page 4. Now something like a .690 roundball chased with seven #1 Buck pellets would be Legal.