I am not sure because ideally, the easiest and healthiest way to lose weight and reach their ideal BMI is by losing about 2 pounds a week. In my opinion, it would take a year to completely lose the 3 stones WITHOUT starvation diet, WITHOUT being cranky, WITHOUT burst of food cheating binges.
The most realistic would be losing A STONE AND A HALF in six months. This could EASILY BE DONE THROUGH food proportion control, eating the right foods, and even treating yourself once in a while, and minimal exercise like brisk walking and jumping jacks.
In answer to cre8iveOne ... 3 stones is equivalent to 14 pounds in weight.