Taking calcium supplements is a good idea if you don't consume dairy products and vitamin D will help your body use the calcium.
Multiple vitamins ensure you get all of vitamins and minerals you need every day, which is good if you are on a special diet or if you're a picky eater.
Omega-3 fatty acids are naturally found in fish, flax, soy and walnuts, so omega-3 supplements help if you don't eat much of those foods.
If you're thinking about taking any vitamins or dietary supplements for specific health conditions, you need to speak with your health care provider first. Although vitamins and other dietary supplements are generally safe, taking large doses of some supplements can be dangerous under certain conditions.
More about Dietary Supplements and Safety
Dark leafy greens is a better choice for calcium absorbation. The human body is not able to absorb the calcium in dairy properly because of the calcium-magnesium ratio.