Cassandra - Cassie/Cass
Alessandra/Alexandra - Allie/Lexi
Katerina/Katalina - Kate/Katie/Lina
Angelina - Alina/Lina
Letitia - Letty (my favorite with Isabella and Vanessa)
Christiana - Christy/Tia/Tiana
Georgina - Georgie/Gina
More great suggestions!
Evelyn has been really growing on me lately. I love the nn Evie. I also love Charlotte (nn Lottie) and have a soft spot for Ophelia. I also really like Elianna with my girls' names.
For boys, I love Leonardo (nn Leo), William (nn Liam) and Charles.
For boys, although I really like Phineas and Edward, I cannot use them. If I use Phineas, then all three of my kids' names will be from the show Phineas and Ferb. If I use Edward, all three names will be from Twilight. I have already been asked if the show or saga inspired my sibset! haha
I like the idea of keeping the double letter theme going with a 3rd girl. Charlotte, Julianna, or Juliette are suggestions that I think work really well with your sibset. Some other possibilties:
Isabella, Vanessa, and
Bettina (Betti/y, Tina)
Camilla (Cami, Milly, Milla)
Glorianna (Glori, Gloria, Anna)