You observe an exponential decay because you want to measure the photon of the second reaction of the gamma cascade after measuring the first reaction. However, since the cascade happens almost instantanously (two photons emitted very close to eachother), you are less and less likely to observe a second photon (not necessarily the second photon from the same decay but usually it is) the longer you wait after the first photon.
1-2 has a lower count because 2 can only measure photons in a direction perpendicular to the (first) photons measured by 1. On the other hand 3 only measures (second) photons parallel to (first) photons measured by 1. Since the nuclei are more likely to emit photons in the direction of their spin the (first) photons in 1 are more likely to come from a spin parallel to the axis of 1 which is thus also parallel to the axis of 3 which makes it, for the same reason, more likely to emit a photon in the direction of detector 3 than in the direction of detector 2.