If you connect the additional speaker to the positive terminals on the right and left speakers it will respond the difference between the channels, not what is common between the channels.
The method in the link provided by transmaster will yield the sum of the right and left channels, although it may have a negative impact on the sound quality, especially if the center speaker contains a crossover network.
Note that it is recommended that the center speaker be the same as the right and left speakers and this may not necessarily be the best choice for the center channel information.
I built a simple active circuit to sum the left and right channels and used this signal to drive a monaural amplifier. I incorporated a band pass filter to limit the frequency extremes, since the sum of the right and left channels is full range.