There is no adjustment in the center, only at the two ends.
The center is just a pulley that moves forward and back when the brake handle is pulled.
You should have two jamb nuts (#3 on the diagram) on the threaded end fittings at each of the rear calipers. To take the slack out, you would have to push the metal threaded sleeves (#4) away from the calipers, and then tighten the sleeve (#4 & #6) with the jamb nuts.
There might be a way to shorten and recrimp the cable, but I wouldn't want to do that, and you probably don't need to anyway.