1.5 cups of feed doesn't seem like much for a doe feeding triplets. I would slowly increase her grain intake and see if that helps. Also, at that age, those kids are sucking down the milk. I would separate the kids over night like Neicie suggested and see what you have in the morning. Those kids do the hit and run method of eating and unless you watch constantly, you won't see them. I very seldom see the kids suck (about the same age as your kids) but I know they are getting plenty of milk. My does production has dropped, because those now huge kids are getting it all. I have enough for the house from my 6 milkers, so I don't bother separating the kids at night, and the bucks are leaving in about 3 weeks anyway, so I don't mind the small amount of milk right now. I did separate one of the does form her kids last week for the day and wow, did I have milk at the next milking.
DragonLair Farm in Central Maine. Nubian, Lamancha, Oberhasli, and Mini dairy goats, 2 horses, 8 dogs and not sure how many chickens.