I wouldn't chance going down below 4 Ohms once you bridge the channels on an amp. Couple that with the fact that you're wasting space with a large amplifier whose channels you wouldn't be using, your best bet is probably one of two things:
-Run just two of the subwoofers off of a bridge, down to a 4-Ohm load, and then put the second amp on the unused channels 3 and 4. You won't get much power to it, but it's better than nothing, or you could just leave the third subwoofer out.
-Trade out your amplifier for two smaller ones. Consider a good 2-channel for the front (or if you want more than 125Wx2 then get yourself a 4-channel and either bridge it for your components, or get one with built-in bandpass to run the correct wattage to each transducer), and then a decent Class D mono for the subwoofers that's 2-Ohm stable.