There are many excellent books that are popular among third graders. Here are a few notable examples:

1. "Charlotte's Web" by E.B. White: This classic children's novel tells the story of a young girl named Fern who saves a runt pig named Wilbur from being killed. Wilbur is taken in by Fern's uncle, who raises him on his farm. Wilbur becomes friends with a spider named Charlotte, who helps him avoid being slaughtered by spinning words into her web. The book is a heartwarming tale of friendship, loyalty, and sacrifice.

2. "The Magic Treehouse" series by Mary Pope Osborne: This popular series follows the adventures of brother and sister Jack and Annie as they travel through time and space in their magic treehouse. Each book takes them on a new adventure, visiting different time periods and locations, such as ancient Egypt, the Middle Ages, and the rainforest. The series is designed to be both entertaining and educational, introducing children to historical and scientific concepts in an engaging way.

3. "The Giving Tree" by Shel Silverstein: This classic children's book tells the story of a boy who grows up with a tree that provides him with everything he needs. The tree is happy to give the boy its leaves, branches, trunk, and eventually its very trunk in order to fulfill the boy's needs. The book is a moving tale about the importance of generosity and the consequences of taking things for granted.

4. "Harry Potter" series by J.K. Rowling: is a fantasy book series that is highly popular among children and adults alike, it follows the adventures of a young boy named Harry Potter who discovers that he is a wizard and attends a school of witchcraft and wizardry. The series is full of magical creatures, spells, and adventures that take Harry, Ron, and Hermione through the seven years they spent at Hogwarts. The books are a great way to introduce children to the fantasy genre and encourage a love of reading.

5. "Diary of a Wimpy Kid" by Jeff Kinney: This illustrated novel is written in the format of a child's diary and follows the story of Greg Heffley, a middle school student who is navigating the ups and downs of school and family life. The book is filled with funny and relatable moments that will make children laugh and connect with the character.

These books are popular among third graders for their relatable characters, engaging storylines, and valuable lessons. They are also a great way to introduce children to different literary genres and help them develop a love for reading.