It is only just possible but you will seriously damage your health/body in the process if you do manage it.
I would like to say as well try not to worry too much about actual 'weight'. It is body fat that is important, as muscle and body shape can determine extra or less weight. That is why the BMI is completely inacurate.
But if you want to go with this weight method, then 3 months is a good time frame to work to, but I would aim to lose about a stone in that time.
There is only one way to do this healthily though, eat the right diet and excercise. It's actually quite simple.
The current reccomended healthy eating plan is four or five small to medium meals a day, each one (apart from breakfast) should contain some carbohydrates (rice/pasta/potatoes etc) some protien (chicken, fish, etc) and lots of vitamins minerals and antioxidants (fresh fruit and veg). You can pile your plate high with as much veg and snack on as much fruit/fruit juice as you like. Just keep one day a week as your 'treat day' where you get a takeaway/pizza/whatever.
Also excercise is essential. Cardio excercise will not only keep you healthy, increase your lung function and decrease your resting heart rate, it will also burn off a lot of calories, keeping you slim.
Weight training is also important, use light weights with lots of reps and you will tone and define your muscles without 'bulking up', which will keep your body strong and supple, but also increase your metabolism so you burn off even more calories even when you are resting!