That would surely help dealing with the effect of bumpy ground on chain tension: the first and second pulley (as seen from the travel direction of the chain) ca

That would surely help dealing with the effect of bumpy ground on chain tension: the first and second pulley (as seen from the travel direction of the chain) can be freely positioned relative to their pivots for balance, without affecting the distance between sprockets and the pulley responsible for lateral chain positioning. But two-pulley designs solve some of the radial movement required for the "selector" pulley (the one responsible for lateral) to deal with a wide range of sprocket radii by the offset between pulley and cage pivot: The line between upper pulley and pivot is at a considerable angle to the line between pivot and the other pulley. In a three pulley setup, I'd expect everything to become far too unpredictable if they still kept the selector pulley offset from its closest pivot. If balancing two pivots with a B-screw is difficult, imagine having to balance three pivots with a "B" and a "C" screw! And I'd imagine the impact of imperfect balance to be far worse in a three-pivot situation... (with two pulleys/pivots B-screw adjustment is more nice to have than requirement)