Just to clear up any confusion. Triple pass does not add surface area or volume. It just means the radiator has internal baffles that make the coolant take a circuitous path within the radiator. The increase in efficiency is a result of the coolant spending more time in the radiator. Multiple cores actually add internal (not external) surface area by placing another internal radiator structure(core) directly behind the first. Of course each subsequent core will be less efficient than the one in front of it. But they always add volume.
In the picture below you can see the two stacks of tubes or "cores".
Of course there is also tube size and fin density but that is just getting into the weeds...
