If your end carb's aren't leaking, I'm with Pat, you have the idle screw in to far and your to high on the idle slots, if your you can't set your idle by adjusting your mixture screws, you'll need to drill the mixture screw holes a drill bit size bigger(If you carb's mixture screws adjust air, and not adjust the fuel mixture), to allow more air in (you may have to do it a couple of times to get the air flow you need). set your butterflies with a feelers gage and don't touch it, set the idle with the mixtrue screws.
That is if your not leaking in the end carb's anywhere, like Mike say's.
I have run 3X2's for years, and never had problems, and everyone I knew couldn't get them right & I would fix them for them.

Set them with the linkage off, then after they idle & run right on the center carb, Then put the linkage back on and see if they still run right.

Pat