I believe the 3i uses an impeller pump. I'm not sure how the unfiltered water flows across the back of the cartridge, but this is the limiting factor on cartridge style filters. There's little/no room on either side of the cartridge, so if you fill the whole area with floss, sponge, etc instead of the cartridge, you severely limit the area that gets hit with big chunks of waste material, thus causing it to clog and slow flow much quicker than the factory setup.
Another option is strip a spent cartridge down to the plastic frame and re-wrap with your own material.