First condition score your girls, most goat books will have a description of the process. Most heavy milkers should be a 3, notmore than a 3.5. When did your goat kid? If she had triplets this spring and stil lgives you around 3 ltrs per day I would not expect her to have lots of weight on her back!
A good mix of diary nuts,sugar beet shreds (I feed mine soaked), oats and barleywill be be good for her. Try not to overfeed her on concentrate - all you will get is soft poo... Lots of branches (easy on the ash though), fresh grass when outside (or cut it for them to eat instead of hay). Frut and veg (carrots, apples, pearsmarrows etc) also good.
Mine get Caprivite, seaweed and garlic about once a week (or whenever I remember). also try and find out of your soil is deficient in any thing (your next door farmer will be able to tell you if you have any of the usual ones - cobalt or copper) and you can remedy with a drench if necessary.
But give her time, and I would continue milking unless she is really too thin. Remember - have you ever seen a fat dairy cow (as in a Friesian?) They are meant to be on the thinner side.