Tara,
Some people do lose 3 stones in 2 months but the average weight loss quoted for CD is a stone a month. I think it will depend on your starting weight (the more you have to lose the quicker it will come off at first). Realistically I would anticipate losing 2 stones if you adhere strictly to the diet and count anything above that as a bonus.
Even if you don't achieve 3 stones by then you will have lost a good chunk of it and, let's face it, you won't lose 3 stones on any other diet.
Good luck to you.