Answer:
1. It
2. We
3. They
Explanation:
1. "The book" is talking about a singular object. So, we need to use either he, she, or it. Obviously, books are not referred to by female or male, so we use the pronoun "it".
2. "Sanjay and I" is a plural form that includes oneself. So, we use "we".
3. "Frogs and elephants" are multiple objects. However, we don't refer to them by any gender, so they are simply "they".
Hope this helps!