Select by material

Hi guys,

i have a slight problem.

i'm trying to make a script that performs a few functions based on a gui, I have all of the functions down except one:

i need to collect all of the materials in the scene
then replace the first three materials with a specified set and all the rest with another material
after this i want it to select the next set of 3, apply the same materials as before and invert this selection to apply the other material to the remaining materials

and so on until all materials have been used.

i'm not really sure how to start on this, any tips would be greatly appreciated.

thanks!