OP I had the 'guilt' of adopting a 3 year old when my dd was 9, she was my PFB one and only for 9 years! But she wanted a sibling and we wanted another child, so there we were.
If you were working full time and are now home full time I cannot see how she can possibly have less of your 'presence'. Maybe it feels like she has less time as babies are taking it. Please be careful not to feed that idea, but instead to challenge it and find ways to share your time.
"How do you parents with lots of little ones arrange your day to allow attention for all of your children?" I don't have lots of children but I did need to juggle expectations. We did a lot of things in the early days that were fun for dd. We went swimming, or to the park etc, we watched movies snuggled on the sofa, we did arts and crafts. Some of this will not be possible yet but you can get your dd to help you make finger prints of her and the babies (using coco powder or something safe, please check, it's a long time since I had a new baby!
Get dd to help you photograph her and babies. Snuggle up together with babies in their carry cots and dd on your lap. Dd gets to choose a movie as she is a big girl.
Get dh to take the twins upstairs while you and your daughter do something in the garden.
In corporate your dd into cooking time (safely) (by cooking I mean anything, tearing up a salad, tearing up a head of broccoli, buttering a sandwich, you don't need to use knives or expose her to fire!). Exercise videos etc. Things she can do as a big girl! But only if you are doing them too!
Always say how wonderful and special your time with her is. Don't tell her you feel guilty or bad, say you feel delighted she is your daughter.
"The twins are EBF too, so I'm just stuck to them most of the time, and can't get us up and out and playing like I used to." It won't last forever and she won't remember it all. My 7 year old doesn't really remember being 3.
Take plenty of photos of just you and her. Just her and her dad, her and the twins etc.
"Am I overreacting to a perfectly normal and healthy part of her childhood?" Yes.
But it is understandable.
"Because at the moment I'm feeling so upset that my PFB isn't my just PFB any more!" Of course she still is, but she is also one of three and a big sister.
Don't forget a ton of hormones are sloshing around your system. So feeling bad/sad/mad or whatever is all pretty normal.
Don't pass your anxieties about this on to her. If she says anything face her questions or thoughts when/if they come. But don't write the script for her.