I have tried connecting the 3rd speaker located in the center in series with the stereo pair - jumpered to positive speaker pole of one (say left) and negative to the other (say right) speakerI wouldn't go about it this way, if I understand you correctly the left channel has two 5Ω loads in parallel which means the combined load for that channel is 2.5Ω, depending on your amplifier this might not be a good thing. Also depending on the design of the output stage you're either just sinking the current from the 3rd speaker to ground via the right channel, or worse (if the output is bridged) then you're 'pushing' the drivers with one channel and 'pulling' from the other which is unlikely to sound good.
I'm afraid I've not come across 3 channel stereo before but I do remember reading that crosstalk between the left and right channel can in some cases appear to widen the sound stage. To emulate that you'd only need a low level signal going to the centre channel. You could also adjust the toe-in of the stereo speakers to widen the sweet spot.
I suggest that you use the minidsp before your main amplifier and use a spare amplifier to power the 3rd speaker. In the software you'll need to mix the left and right channels down to mono for the 3rd output (the input to the mono channel should have the left and right set to -3dB to prevent clipping). This means you can adjust the phase, level and frequency response to both the stereo pair and the 3rd channel.
Hope that's helpful.