Uhm no the 3 channel is its own independent line of signal.
You must have an AV tool that can at least do multichannel audio even if it's just Surround Sound or else there's no need for the centre channel signal for an amp @jbuhl
3 channel amps
I assume 3 channel amps are for running a center channel? Is the 3rd channel a combination of left and right? Looking at a used Classe CA-3200