Could you please explain the three difficulty pools and what conditions would you recommend for selecting a higher difficulty over the lower one?
Sure, if you are only running a say 1-4 graphics cards, then the lowest difficulty should suit you fine.
Then say ~4-15 the middle should start you out pretty well.
Above that go the highest.
No matter which one you chose, vardiff will kick in and settle you on an appropriate speed for your setup..