Most manufacturers now offer a feature on their amplifiers known as "Multi-Mode". Kenwood calls theirs Tri-Mode, and other manufacturers have similar pet names. Basically, multi-mode allows you to run an amplifier in two channel and bridged modes at the same time.

This capability is actually inherent to most bridgeable amplifiers on the market. You can run three identical subwoofers if the amplifier is capable of handling low impedance, or using a special crossover, you can run a single sub and two pairs of full range speakers.

Here is how it basically works. For the most common usage, you will need some basic crossover elements: Two capacitors, and a coil. For our purposes, we will use 2 99uf capacitors, which crossover at 200 Hz for a 4 ohm speaker, and a 6.4 mH coil, which will crossover at 200 Hz for a 4 ohm speaker (if you are unfamiliar with how crossover elements work, go back and read up on it). We will also assume that we are using a Kenwood KAC-921 amplifier because I said so. Run wires from each channel to their respective speaker. Place the capacitors inline with each speaker (for simplicity sake, do it on the + leg of the wire). Then, run your wires from the bridged outputs on the amplifier to your woofer, with the coil inline. In this method, you have created a simple 6db/octave crossover network. Our amplifier is rated at 100 watts/channel RMS. Because the crossover network separates the frequencies, the power is not shared among all three speakers at all frequencies. For bass up to 200 Hz, the sub will get 200 watts RMS and then it slopes off. The mids will get 100 watts each.

In the above example, each driver would get 100 watts. It would also sound like shit. It's important for the crossover to be there, especially for this amp, in order to prevent the amp from being overdriven.

Now earlier, I said that any amplifier that is bridgeable can be run in multi-mode. While this is true, it is important that you understand why. If you try to run an amp in multi mode incorrectly, you can easily damage it. For starters, if your amp is not rated at 2 ohm bridged, then do not try multi mode for three woofers. You can still run a woofer and two mids with a crossover, because the crossover keeps the impedance up for specific frequencies. Also, if you plan to use the multi mode feature to run a sub and two speakers, you must have a crossover to realize maximum performance.