In the 3 blade prop area, you have broken up the air with the previous prop a bit more so, the thicker the air, the more effiecient the prop, it's kind of like putting 2 motors and props operating independantly just behind each other, the rear prop only has the left overs from the front one, this is why you don't see them too pushed with RC plane designs. The same principal is demonstrated with RC combat where the front plane prop washes the other, causing the opponents plane to fall out of the sky since it doesn't have dense enough air to pull in enough air to transfer to it's prop, that and of course the instability caused by the wash.