Roadkill, are you thinking of doing away with the engines' tanks and using a central feeding tank?
Unsure if engines will suck that far, especially since the tank might be lower than the carbs.
Besides plumbing THREE fuel inlets, you'd have to hook up THREE fuel return lines to the central tank.
Buying the $60 worth of engine tanks might be a better idea. You wouldn't have to worry about fuel return lines routed to central tank.
You'd also have additional two quarts' capacity using engine tanks.
Ya might need THREE fuel valves to isolate individual tank feedings. Pet shops might have brass acquarium valves that might work.
THREE engines would need THREE throttles unless you use a dual-pull lever and synchronize two engines.
Also unsure how two different engine systems on the same rear wheel would perform.
Sounds like a project. Try one engine install, then the second then the third.