I am a great proponent of walking for fitness. Walking has the advantage of not requiring any equipment and carrying the least risk of injury of any aerobic exercise. Use good posture, swing your arms, and keep a good pace. Three miles should take about 45 minutes. Specific forms of exercise have their own benefits.
Swimming is great for the joints, a balanced muscular workout, and relaxation.
Cycling builds knee muscles and can provide a feeling of exhilaration. Dancing is one of the best aerobic activities of all, because it's fun, never boring, and provides a thorough workout. Once you have developed good habits of regular exercise, begin to add stretching, muscle toning, and strengthening to your routine. Yoga is a great way to stretch, improve flexibility, and experience deep relaxation. Breathing exercises, meditation, and other forms of relaxation are important to help neutralize stress.