If your exercise plan includes lifting weights three times a week, here's a great workout that's effective on your time and your muscles (don't forget it's 2 seconds up and 2 seconds down):
Push-ups (works chest) Do as many push-ups as you can, positioned either on your toes or on your knees.
Squats and Rows (works glutes and upper back) Stand with feet hip-width apart, heavy weights in each hand, palms facing behind you. As you squat down (keep your knees over your ankles and don't go deeper than 90 degrees), bring your fists to your armpits while squeezing your shoulder blades around your spine. Pretend you're trying to get your shoulder blades to touch each other. Release the weights to your side as you stand straight. (3 sets of 10 reps)
Lunges and Curls (works hamstrings, quads, and biceps) Stand with feet together, heavy weights in each hand. Lunge forward with your right foot (keep your knee over your ankle and at a 90 degree angle). Your left knee should be just above the floor. When lunging down, curl your biceps by bringing your weights to your shoulders. Release your bicep curl as you bring your right foot back to your left foot. Now, step with your right foot backwards and do a backwards lunge as you complete another bicep curl. Return to starting position. 1 rep will include both a front and backwards lunge. (3 sets of 5 reps for each leg)
Plies and Extensions (works glutes, calves, and triceps) Stand in a very wide stance plie position, toes pointed out, with a little squat. Your elbows are bent and next to your ears, medium weights in your hands, palms facing each other. Extend your weights above your head as you stand on your tiptoes. Return to starting position. (3 sets of 12 reps)
Push-ups (works chest) Finish this workout with what you started. Take the number of push-ups you did the first time and add at least 3; complete that many push-ups, and then you're done for the day!