On my setup, the center carb linkage controls the front and back carbs. The center opens about halfway and then opens the remaining two. The further you get your foot into it, the more open all three become. Front and rear are in sync with each other, while the center one only taps into the front and rear at a given point. On my system that point is adjustable.

It can be adjusted to open all three in sync or to never open the front and back and just run off of the center alone. Runs a little rich that way but can be done.

Another thing I guess I need to mention is that my linkage was home built not purchased, as several modifications needed to be made, and the store bought linkage wasn't up to the job. I think I still have the a progressive linkage setup in shrink wrap, Offenhauser I believe.