Description: Circuit training that includes exercises that combine cardio and strength training in short and varied sets of each, so it can help you build muscle and lose weight. 3-2-1 is named for its three phases, 3 cardio, 2 strength training, and one core training. Each set includes the following pattern of workout and there are four sets total: Cardio, Cardio, Cardio, Strength, Strength, Core.