Hi and welcome to COL

and well done for taking on your rescue girlie. We're a very nosey lot on here, so it will be lovely to hear more about her and see some photos of her too please

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Raw green tripe (absolutely stinks, but most dogs love it) is a good food for putting weight on and is also full of natural enzymes, so it's kind to the digestive system. Personally I wouldn't feed it in the same meal as kibble because raw food is digested quickly whereas kibble takes a lot longer to digest so mixing the 2 together can cause problems. You could try feeding a meal of raw green tripe in the morning and one of kibble in the evening.
If you decide to do that the easiest tripe to obtain is probably Nature's Menu, as most Pets At Home and many other pet shops sell it. Or you could go on the Natural Instinct website and see if there's a local stockist. You can order online from them, but if you just want to buy some to try it will be very pricey to buy it online.
Raw dog food is generally fed at between 2% and 3% of the dog's ideal bodyweight per day. So to give an example, if your girl's ideal bodyweight is 12kg a full day's ration would be between 240g and 360g per day. So for feeding 1 meal of tripe per day I'd suggest 1.5% of her ideal bodyweight.
If you decide not to feed anything raw and stick with kibble I'm not sure I'd feed her 3 times daily allowance as that's such a large amount of food that I'd worry it would put a strain on her system, especially after having been so poorly fed, but would look to make a steady weight gain on a more normal amount of food. Did your vet advise to feed 3 times the amount? If so that's fine of course (I'm not a vet or nutritionist) but if he didn't say that then it's probably best to check with him.