i farm circle of fire on underworld madness...per 80 sand of time i get approx. 1-2 3* rainbow runes (and any other colour).
when i try any other world, the rate seems to be much lower. played yesterday battlefield madness the boss-stage and no 3* runes in approx. 25 tries.
maybe this was bad luck, but circle of fire is my bread and butter level for runes sice weeks now