This exercise follows on from the interval of a 6th, which occurs at the beginning of bar 30 and is an exercise in dealing with wider intervals.
Much of the information from Exercise 2 will be helpful when practicing exercise 3b. Once again, always try to follow the dynamic markings and make shapes. This exercise is also useful for finger discipline in the top octave, where movements between notes at speed can be awkward for the hands. It will be worthwhile playing this exercise slowly.