There is a toning class at my gym that uses 3-lb. dumbbells. Will I get the same benefits from this class as the StrongWomen programs?

Exercise with relatively light weights will burn calories and may increase endurance - and like all exercise, it's good for you. However, you will need to lift weights that are heavy enough to feel challenging so that you can build and preserve muscle and bone, and to get the metabolic boost of having more muscle.