Zoology 1 covers the classification system and binomial nomenclature, which are used throughout the three zoology texts. It also introduces terms such as extinct and habitat. The subsequent texts assume that the student knows these things.  You can read Lesson 1 of Zoology 1 to learn these concepts (except for extinction) before moving on to Zoology 2 and Zoology 3. Then it really doesn’t matter which order you read the books.