For the sake of simplicity, lets assume that the radiators are equally thick, the fin density is equal, and they're made out of the same material.
120mm * 120mm = 14400mm^2
140mm * 140mm = 19600mm^2
200mm * 200mm = 40000mm^2
This means that a triple 120mm radiator has 43200mm^2, a triple 140mm has 58800mm^2, a single 200mm has 40000mm^2, and a dual 200mm has 80000mm^2 of surface area.
In this case more surface area = more heat dissipation, so the dual 200mm radiator is easily the best one.