Real tractors are big, loud and dangerous, but models can be cute and colorful. Have your child build a model tractor out of a soda bottle. Make sure to include each part so they are really learning. The wheels can be large and small bottle caps and the smokestack can be a straw. It's fun and easy, and you're also recycling. Your child can also create a diorama of a farm or construction scene.
Just like pilots love when kids ask to see the cockpit on a plane, adults usually enjoy talking about their jobs to kids. Find a local farm and go on a visit with your kids. Talk to a farmer and have her explain why tractors are so important to the industry. To take the project to another level, your child can write a report on what the world's food supply would be like without the use of tractors. If you live in the city, then you're probably close to a construction site. Keeping them at a safe distance, take your child to a building site and let them watch the machines at work. Let your kid take pictures and talk to some of the workers and write a journal about what they saw and learned.
You may not have time to drive to a farm or construction site, but odds are, if you have a son, then he owns a toy tractor or bulldozer. Have your child go out to the yard and reenact what tractors really do. You can help catalog the event by taking digital photos. Print out the photos and your child can put the photos on a poster board and show his class what he has done and learned.
If your third grader has an artistic side, encourage it. A drawing or painting of a tractor will develop her dexterity and spatial skills. Plus, it's fun, and you can display the masterpiece on the refrigerator or wall. It's also safer and easier than the other projects if you're short on time. To add more, your child can write and illustrate a short story or children's book where a tractor is the main character. With this, your child can be artistic and learn about tractors at the same time.