You only need 4 of the 3-way crossovers: FL/FR/RL/RR

Basically, for each crossover, your amp output will go to the input for the crossover. So for example, the amp FL output will go to the input for the FL 3-way crossover. From the FL 3-way crossover, you will then have 3 output to each of the FL component speakers (e.g. tweeter, mid, and mid-bass). Repeat for the FR, RL, and RR speaker sets. For the rear, since you only have 2 speakers for each rear side, you only need to use 2 of the outputs from the crossovers for the rear (e.g. mid, and mid-bass). Hope that helps.

Oh, btw, you can use 2-way crossovers for the rear RL/RR speakers; however, the frequency range for each of the 2 channels coming out of the 2-way crossovers will not match the ranges for the mid and mid-bass speakers. That's why I used the 3-way crossovers for the rear sets as well.