Given that she may already have a lot of casseroles and the like from other people, you might consider bringing something like fresh fruit. It can be expensive, but if you can swing it, they might enjoy some fresh food. Or milk. The sort of thing that you need to go to the grocery store for when your baby is 4 days old, even though people have been bringing you food (yes, I speak from experience).
If you want to bring a gift for the babies, a book (or books, but I don't think it's necessary to have exactly 3 just because they're triplets--they'll share books) would probably be welcomed. Clothes are nice, especially in larger sizes, but moms tend to get a lot of clothes and toys, and sometimes a book is a nice change. I love the "That's Not My" books by Usborne ("That's Not My Truck," "That's Not My Teddy," etc.). And they're a little different from the "Good Night, Moon" kind of books that they might already be getting duplicates of.