Bold words in a story at a grade 3 level usually indicate important vocabulary that helps convey the main ideas, themes, or characters in the story. These words might be new or significant, and the bold formatting draws attention to them so that readers can focus on their meaning. Often, they are words that the author wants to highlight for better understanding. Sometimes, bold words may also be defined in context or in a glossary at the end of the book.
If you have specific bold words in mind, feel free to share them, and I can help explain their meanings!