If you need to lose 3 stone, you really should see a GP before you start any diet or weight loss programme.
They will probably refer you to a weight-loss clinic who will advise you carefully to follow a programme that will help you lose weight and maintain your lower weight.
Be aware that most people who start dieting then yo-yo up and down for years, frequently ending up at higher weights. The rate of loss can be part of the problem - losing faster normally is followed by putting the weight back on - and losing 42 pounds in 13 weeks (3 lbs/week) is quite fast enough.
If you are serving flightcrew then your varied working/resting hours/sleep pattern will have a huge amount to do with your problem - so you will need to look at how you deal with that while you lose weight.
Good luck