三年级下册英语第三单元的几个句子的读法如下：

1. "Is this a library book?" —— 读法：This a library book?

2. "No, it is not. It's my book." —— 读法：No, it's not. It's my book.

3. "Is this your pencil?" —— 读法：This your pencil?

4. "Yes, it is. Thank you." ——读法：Yes, it is. Thank you.

5. "Whose book is this?" —— 读法：Whose book is this?

6. "It's a library book." —— 读法：It's a library book.

7. "Do you like books?" —— 读法：You like books?

8. "Yes, I do. I like library books." —— 读法：Yes, I do. I like library books.

需要注意的是，最好在教材和母语人士的指导下准确学习语音发音，并勤加练习，才能在后续的英语学习中更好的掌握口语技巧和沟通能力。