Congratulations on 3rd baby im on my 3rd too. I've been trying to watch the weight gain too so far ive been ok.
I think as for sw plan just follow plan normally but I think you do need extra healthy extras for fibre, calcuim and victamins. Once you told your consultant she should give you a booklet on sw in pregnancy which give you more info also ur need to get permission to carry on during pregnancy from midwife. There is a slimming world and pregnancy thread you may want to have a look at, the girls are so friendly and really helpful x