The triple setup makes much more sense than any dual carb arrangement, since each carburetor can be sized to supply the proper mixture for a single cylinder. Even with siamesed ports, the intake impulse is 360 degrees of crankshaft rotation apart for each port. Basically, with the right size carbs, triples are the equivalent of six one-cylinder engines sharing a common block and crankshaft. Duals, not so much!
The S-U also has a "constant depression" (vacuum) system where the sliding piston effectively minimizes the tendency to over-carburete and maintains sufficient manifold vacuum for good low speed and midrange performance.
Jerry
