This looks like a quasi-bridge set-up. You're still using the L and R channels to feed L and R speakers in the right phase, but you're bridging the positives of both channels to drive a center speaker. I don't know if there's a voltage potential between the positives of both channels to allow a flow of current to drive a speaker. I haven't tried this, but I recall hearing an audio guy in my college days say this is possible. Like a bridged stereo or summed mono set-up. (I think the 3rd speaker is supposed to be connected to the left positive and right negative, not to both positives.) Where did you get this schematic? If the source of your schematic says it can be done, then you can always try. Start with low volumes first. With regards how the impedance would look to the amp, all I know is that in a true bridge configuration, the speaker load impedance will be halved as seen by each amp. On your set-up, I think the impedance seen by each channel will be halved as well or even lower. Half the third speaker's load will look like in parallel with each L and R speakers, so it could be less than half as seen by each amp. So before trying this out, as a caution, I suggest you use stereo amps that can handle low loads.
