Wow, what a predicament. First of all, if you are pregnant, congratulations! I don't know what your feelings are on another little on coming along in the family, but judging based on already having two little ones already it may not be the most ideal timing, but I'm sure you will make the best of it and welcome a the new one into the family with open arms
I'm looking forward to the day that I will have that blessing myself.
I wish I had some advice for you, but you might want to ask your doctor about a food plan. I've heard before that very overweight women (and it sounds like you're probably about the same weight as me) may not need to gain any weight during pregnancy, or if they do, very little. The doctor should be able to give you some guidance on daily calorie intake, considering that you're pregnant and nursing. Hopefully your body will be using those extra calories and you won't gain any significant weight. After the baby is born then you can tackle the weight loss -- probably continuing to eat some of the same foods, just less of them.
At any rate... good luck. I don't normally post in this particular sub-board as I am neither pregnant or nursing but it caught my eye on the front page