Hi Misscoco!
Average losses for 810 are approx 10/12lb a month, obviously this is an average and some will lose less some a lot more, it all depends on how much you have to lose.
I would think a stone in 3 weeks is cetaintly obtainable, if you stick to the plan 100%.
I would say start off on ss for a few weeks just to shift the weight quickly and see how you get on. If it doesn't suit you, you have the option to move up a plan to ss+ which allows a small meal in addition to your cd sachets or the 810 which is similar to ss+ but allows you a slightly bigger evening meal.
It really all depends on what you are happy to do. Average losses for ss are approx 14lb a month. So the choice is up to you.
Have you actually seen a cdc yet to discuss things? Whatever you decide to do I wish you the best of luck!!