Hey A.! I think if you're wanting to lose some weight that it's best to do it BEFORE you have your next baby b/c as you know, weight loss/diet during pregnancy isn't recommended. And then, if you nurse your baby, you still don't want to be focused on weight loss either.
As far as the amt. gained with each pregnancy - I've had 2 kids and I gained more with my 1st (my son was 9 lbs. & I think I gained 30 something with him [38?]). I did not loose all my "baby fat" after I had him & so I started my pregnancy with my daughter around 20 lbs. heavier. With my daughter, who was 7 lbs. 6 oz. I only gained 26 lbs. I nurse her so I am back down to "around" my weight before I had her and I could still stand to loose more :)
... my sister who just had her 3rd baby gained the least amt. with this pregnancy & this one is her smallest baby.
Each person & each pregnancy is different so weight gain will differ from person 2 person. Alot about weight gain during pregnancy also depends on the type foods you eat.
Good luck loosing the weight & best wishes for trying for a 3rd baby!! :) Have a GREAT day!!