3 times a week is adequate as long as you workout hard and stay consistant. If you have the time 4 to 5 workouts a week is better. For those who are trying to loose weight or tone up working out 3 to 4 times a week is enough.
Most people make the mistake of just doing cardio and ignoring resistance training.
Incorporating weights, cardio, and cross-training is the way to go. For those afraid of weight-training, it's important to realize that when muscle is built, fat is usually burned. The exception is abdominals, calves and forearms which can be trained multiple times per week. When training to gain muscle mass, it's important to get more rest in between sets and don't over-train by working out too long or doing too many sets. I think I should.