3 pairs of speakers

Not the best way.  The high level inputs on the amp are an inferior way to input the signal and is used only out of necessity.  Use the RCA low level inputs on all the amps.  There are some different solutions:

1. Use RCA splitters to send the deck signal to more than one amp.  The second small amp that powers the third row can be adjusted independently of the first two rows through either the use of the amp's gain ( for a set level), or the third row speakers can be adjusted in volume through the use of potentiometers (L-pad).  The best selection of L-pads can be found with a standard of 8 ohm impedance, so if you have 8 ohm speakers in the rear this solution will cost the least.  I have found only 1 4 ohm potentiometer in searches earlier but there are a lot of cheap 8 ohm models ( see selection ).  You can install these controls where the third seat passengers can adjust their relative volume or you can hide them where only you can adjust volume.

or 2. You can install a line driver to increase the number of signal outputs.

Of these I would choose the adjustment of the small amp's gain as a simple and effective solution.  You want to be able to use the deck's fade control for adjustment of the second row speakers without affecting the relative volume of the sub, so the sub amp input should be from the deck's front preout, not the rear.

But my head is hurting today so anyone feel free to correct me if I blundered....:)