If it is brief (4-5 pages) and helps “set up” the book, you may include it in addition to the three chapters. If longer, you should regard it as one of the three chapters. The goal is to help the reviewer understand the concept of your book; usually a good synopsis does that. Our reviewers understand three chapters to total about 20-30 pages for normal prose (not children’s books). Long chapters that total far more than 30 pages will require the reviewer to stop after about the normal 30 pages.