D.B.
It's a common phase and it's incredibly annoying, I know. But you have to play it out. These 2 nieces have been in your family since before your daughter's birth, so she may just be figuring out that there's a difference in parents. But a lot of this comes with actual siblings as well.
I think volunteering in a kid's preschool class is often a bad idea - the purpose of preschool is to get the child to transfer her sense of authority to the teacher, and to work in the group. Having a parent there can be very confusing - it's not home, it's school, but Mom is there. So I would stop that anyway.
At home, I'd try to explain that your heart is big enough to love a lot of people, and so is hers. Families have all kinds of configurations. You might read her some stories about different families - the children's librarian can help you with some suggestions. I'd also have some special time with each of the 3 children. You might incorporate some of the verbiage from preschool about taking turns and everyone being important. School also has story time and snack time and art time - in your family, you have dinner prep time and hair brushing time and her bath/story time.
And when others are available to hold her and read to her, are they not available to help with dinner (Dad can cook, 9 & 13 can set the table with some "help" from the 3 year old). Everyone can help a little. Then it becomes a family activity.
I think, if you can establish some family time vs. some one-on-one time, and some "Mom's alone time" you'll eventually show her the boundaries.
Good luck - it's tough, I know!