Dr. Simeons states that in Phase 3 you cannot eat sugars and starches. Sugar and starches can be hard to determine so read your labels! For more information on sugar read this article on Alternative Names for Sugar.
It is important that you watch starch and sugar religiously during the 3 weeks. Eat anything else you want but know that as soon as fats and starch are combined your weight is very likely to get out of hand.
Potatoes, grains, bread, and pastries are your obvious starches and consuming them can be dangerous. You will also need to watch your vegetables. The main veggies to be avoided are the starchier vegetables such as all potatoes, corn, carrots, squashes, beans, peas, and lentils. Many root vegetables (things that grow underground like potatoes and beets) will be higher in sugar/starch.
It may not always be that easy to see the starches. Many times there will be starches where you don’t expect to find them. Sometimes they tend to sneak starches as a thickener and many times you will find starch in processed food. One example – lettuce in a bag found in your produce section contains starch to keep the lettuce fresher longer! So be careful and buy fresh organic produce and check all food labels!