1.) Run the left three in series for 12-ohms, the same for the right. Bridge a 4-channel amplifier into 2-channels. Use an amplifier that generates its full power into 4-ohms, so you're not losing much of its full rated power into what will be the load equivalent of 6-ohm stereo. An Alpine PDX will work here.
2.) Wire the left three in parallel for 1.33-ohms, the same for the right. Use an amplifier that is 1-ohm stereo stable like the Boston Acoustics GT-2200.
3.) Wire the left three in series, same with the right. Parallel the two sides for a collective 6-ohm mono load. Use the JL Audio HD 750/1 monoblock.
Obviously you can run a six channel amplifier with twelve conductors up the tower but the above three scenarios, and a few other options, will allow you to maintain a four-conductor tower harness.
While the speakers may be inexpensive, you have a combination (six tower speakers) that is expensive to power reliably.