It is possible to lose that much weight in three months, but it's not possible to lose that much HEALTHY weight in three months unless it's almost all water weight and you were very large to start with. Once excess water weight is disposed of, a healthy loss is generally only a few pounds a month...like less than 10, though the exact amount depends on your body composition and the difference between your healthy weight and current weight. If you're losing significantly faster than that, there's a very good chance that what you're losing is muscle. You don't want to lose muscle, you want to lose fat. Muscle is what allows you to burn calories, and is obviously the component that makes you feel stronger. It's also what your heart is made out of, so those fad crash diets and starvation plans can have some extremely serious implications if they're used consistently. Not to mention, muscle weight loss is not permanent...once you get off the crash-course diet and exercise regimen, you will have a reduced ability to burn calories and will likely gain back more fat than you had before. Let the yo-yo begin.
The only way to really lose healthy weight is through a permanent lifestyle change which must include moderation in eating -- but not necessarily completely cutting out everything appetizing in your life -- and regular exercise. It will take a while to lost the weight...it took you a LONG time to put it on. Personally, I made my lifestyle change almost three years ago and never did any of this fad dieting or excessive exercise, just slowly cut down on "junk foods" while adding in more whole grains, fresh fruits and vegetables, and a reasonable workout schedule. Nine months ago I weighed 65 pounds less than when I started (a reduction of 27% body mass, mostly fat with some excess water weight), and was still losing weight. Right now I'm two weeks away from having a baby, so there hasn't been any loss for a while, but there also hasn't been more than the healthy amount of pregnancy weight gain.
For all the people that have tried everything to lose weight, how much did you weigh two years ago? Instant gratification is the order of the day, sure, but that 65 pounds I shed in two years has never come back, and never will. It doesn't take anything crazy, and the crazy stuff is almost always impossible to maintain permanently and almost always backfires on the people who try to use it. Even now, nine months pregnant, I'm just 35 pounds from my goal weight...by mid-2011 at the latest, depending on how much weight I retain after baby, I should be hitting my goal weight and will stay there.