Hi Nathan, and welcome
I've been with Slimming World for a long time, and everyone loses weight at different rates. Some people can lose 2 stone in 2 months, others can lose 1 stone in 3 months. It completely depends on each person. Even following the plan 100%, everyone loses at different rates, so there's no way to guess how much you can lose.
But if you lost 1 stone in a month last time, you could lose at the same rate again, which could mean at least 3 stone. But there is the chance that you could lose at a higher rate, so whatever happens, stick with it.
Losing weight is a marathon, not a sprint. So whatever rate you lose the weight at, just think of it as being lost for good.
I agree with exercising too, normally I try to lose the weight and then exercise (but that's also because I was lazy
lol) but this time I'm doing both.
One thing I will mention is that if you start exercising at the same time that you start to change your food, you may not lose the weight as quickly as you did last time.
It's purely down to a fluid build up in the muscles after you exercise. When you ache the day after exercising, it's because your muscles have torn and they are starting to repair themselves and you get a fluid build up whilst they do this.
Don't let this stop you from doing both though! You should, especially if you are planning to go into the army, it'll help increase your fitness level
Good luck though, and hope to see you around the forums