Sarah, sorry if I was a bit harsh in the other thread.
Something to bear in mind:
You don't weigh 16 stone.
You, your baby, its placenta, its amniotic fluid, your expanded womb, your extra blood, fluid, breast tissue and essential survival fat reserves for pregnancy and breastfeeding weigh 16 stone.
I'm not saying it'll all disappear the moment you give birth, but that number doesn't reflect your pre/post pregnancy body.
Speak to your midwife if you feel you are gaining more weight than you should do about your diet and lifestyle. If your main concern is after the baby is born, then speak to your GP at your 6 week check up (by that point you'll have a better idea of how much You weigh) about long term changes you can make to diet and lifestyle.
I appreciate you'll be a busy mum of three and you might not be able to fit in loads of exercise, but you will be able to add in some bits and pieces to make your daily activities more active. Some people add in lots of little things (like parking further away from the supermarket so that you walk more, or taking the stairs etc), others find it easier to carve out a half hour or an hour to go to an exercise class/run/gym session when someone else watches the kids and they get some me-time.
It is possible to lose weight through diet alone though. I did (before pregnancy) because I find it difficult to get out to exercise. Its often about portion sizes, healthier choices, and being occupied enough not to snack. If you struggle to lose weight, make sure you have a check up with your GP, I have a thyroid disorder and found it easier to lose weight once it was being treated.
Be gentle with yourself and your body. It's growing a whole person right now. :)