SBD is not a quick weight loss program, so I really do not know how to answer that. Do your best and you will be lighter and healthier at the end of 3 months than when you started. But their are no guarantees as to the amount you will lose or the rate at which you will lose. You are talking about 15-18 lbs in US terms which isn't a lot as far as weight loss goes and those last 5-15 tend to go much more slowly than the first 15-18 lbs for someone who has twice as much to lose for example. My last 5 lbs took me six months. Just the way it goes sometimes. I was still far healthier than before I started SB and because I had no deadlines or expectations, I was fine with however long it took.
I'd maybe work with a dietician and/or a personal trainer to design a program that might get you there in that time frame if it's that important to you and then use SB after that to maintain perhaps?