You might be interested in this article by the University of California. http://www.berkeleywellness.com/html/ds/dsChromium.php
Chromium has no effect on weight loss. There are potential dangers.
Calcium has been shown to help in weight loss, if you get the calcium from natural dairy sources and not supplements. It is thought that there are other factors at play to work with the calcium, so you need to get it from the source. Not supplements. If interested, you should get from 1200 to 1600 mg calcium per day from your foods. While this won't result in weight loss by itself, it can slightly increase the amount of weight you lose when you control calories and exercise.
Vitamin C has no effect on weight loss, but it's good to take it if you don't get enough from your diet. Your body will use what it needs and discard the rest in your urine, so it doesn't do any good to take very high dosages.
Hope that helps