One idea for a good third-grade science project is to test how smell affects taste by having 15 volunteers taste various pieces of fruit. For the first round, they'll be blindfolded, the second round they'll have their noses plugged and the third round they'll be blindfolded again.
Peppermint oil is placed on the fruit the first time it's given to the blindfolded volunteers. The volunteers will then test the fruit without the peppermint oil on it, but with their noses plugged. Finally, for the control, the volunteers will be blindfolded again to try the fruit without the peppermint oil. After each round, the volunteers should try to identify the fruit, and this information should be recorded on a table that has columns for the type of fruit and each round. At the end of the experiment, the class should evaluate the results to see how well the volunteers recognized the fruits with the presence of peppermint oil or the lack of smell.