The term 'La Brea Stuff' refers to the TAR pits located in Los Angeles, California. The TAR pits are a natural asphalt deposit that has been seeping up from the ground for thousands of years. The TAR pits have yielded a wealth of fossils from prehistoric animals, plants, and insects, making them an important archaeological site. The three letter solution to this crossword is 'TAR, ' which stands for the TAR pits located in La Brea.