I don't know about stacker3 specifically.
Pills with ephedra (now banned) were effective because it was basically like being on speed...more energy, faster heart rates, etc. I think trouble breathing was reported with pills like these. In addition, they were linked to heart attacks. they worked, but I don't think it's worth the threat.
Other pills really aren't going to be as effective. They generally have lots of caffeine which will give you energy and hopefully help you workout harder, but there is nothing magical in these pills that will make you loose weight. Many of them do contain things such as niacin (vitamin B12) which have been connected to weight loss, but this can be obtained through other methods.
One pill that does sound interesting is CLA. CLA is a supplement that is derived from safflower oil. It's basically a healthy fat that has been shown to improve immune system functioning and it seems that people also lose weight on it, but only if they are dieting and working out anyway (basically it just helps). I don't think it's a fat loss catalyst or anything like that, but at least it doesn't contain anything harmful.
Really a healthy diet with lots of exercise is the only way to really lose wieght and keep it off. Some of these pills might help you, but if you don't learn the proper way to eat and workout, the weight will come back anyway. The only pills I would take would be multivitamins and maybe a calcium supplement. Calcium has been shown to aid in weightloss although the pill form is only about half as effective as getting it through food - plus eating a yogurt is a lot better then cookies or ice cream!
Feel free to send me a message if you want some tips to assist you in finding a diet and exercise plan that will be effective for you!