I cannot speak with absolute certainty but every multichannel amp I have ever owned (and none of them have ever been PPI gear whether before or since the DEI buyout) required the following for bridging.
Your positive input on the sub need to be connected to the left amp channel's Pos (+) output.
Your negative input on the sub needs to be connected to the right amp channel's negative (-) output.
Whether that amp will work good on a single 3 Ohm driver when in bridged mode I couldn't say.