Now you take these 2 hands facing each other and turn them palms-down so that they form one line of 8 fingers, a permutation of the original set-of-8 which happens to have a piece of tape (or other division) separating the first 3 from the last 5. Ignoring this division you have just the permutation of 8.
With a little reasoning you get the one-to-one and onto properties of this mapping: if you got to some permutation of the 8 this way, there was only one choice of the 3 out of the 8, and only one ordering of the left, and only one ordering of the right, that generates that permutation of the 8. Furthermore every permutation can be generated by those very steps of choosing the first 3 of the permutation in the 8-choose-3 step and then ordering the two groups appropriately. Since this process is one-to-one and onto, it must be the case that (N choose K) K! (N - K)! = N!, the total number of possibilities for each must be the same.
Does that help?