Anna, Cato, Dana, Lena, Luna, Quinn, Sara or Suze

It could be my own list, what lovely names!
Anna: classic, quite pretty. Fits a bit less with Mila and Nova, because I find it a completely different type of name.
Cato: One of my favorites! Really nice name, but same as with Anna, it's a different style than Mila and Nova.
Dana: pretty! Also pretty in English (pronounced Deena).
Lena: nothing to complain about, good name. Not such an exciting name (sounding boring sounds too negative, but I'll describe it like that for now), but that's not a big deal.
Luna: I think this fits the style best with Mila and Nova. Good name, but in my opinion it's been around a bit too much lately.
Quinn: tough, cool!! But maybe some confusion about whether it's a girl or a boy. And different style.
Sara: same as with Anna, pretty but different style.
Suze: see: Sara

Other options:
Mila, Nova and...
Tess Riva Pippa Rosa Yara Evi Liv Liva Mara Mare Maire Maeve Deena Romee Jonna Yora Sari Ylva Ilse Lois

I’d love a girl’s name ending in -a, since both Mila and Nova have that.
From your list, I think Luna or Lena fit best.
Anna and Sara are also beautiful, but a bit more chic than the more modern names of the sisters.
Is Vera maybe still nice? Or Juna / Tara?

My daughter is named Anna! And I can tell you, I've received super positive reactions. Everyone who heard it for the first time said what a beautiful name, whereas with my first child it was only occasional. Yes, it's a classic name, but at the same time quite modern, because you hear it more and more. And personally, I think it suits the other two names of your girls well.
I also think Lena is a beautiful name, but I'd make it Elena, it makes it just a bit more special.

I find a name without an 'a' at the end or with more or fewer syllables just cooler. So I went with Cato. Not Quinn because of the boy/girl confusion. My friend got a few comments like 'oh, lucky, now a boy!' when her third girl had a unisex name too. Not cool when you're just proud of your girl.