An in expensive way to add some speakers and keep the load the same at the amplifier is to use a 1st order xover on the speaker. This is simply done by adding a capacitor to the positive side of each speaker. This will change the load the amp sees from a resistive to a capacitive, effectively fooling the amp. The value of the cap should be chosen by the amount of bass you want to pass through to the speaker. The higher the value, the more bass that will be played by the speaker. I would consider about a 470 microfarad capacitor that is designed to operate at voltages over 12V. This will allow plenty of mid bass. It will not effect the mid or high frequencies. The down side to this is that the speaker will be able to play only mid bass and up. The up side is that by wiring a 3rd set of speakers in this way, you add cone area, adding volume without putting a strain on the amp. I would simpily tie the 3rd set to the rear speakers in parallel. Even a decent aftermarket radio can drive this load.
G