New heights! How do I deal with this crap?

Basically I have three employees, let's call them Jane, Elyse, and Donna. I'm firing Jane because she doesn't do anything at work, and isn't at all responsible. Jane is friends with Elyse. Elyse is hardworking and treats Donna horribly. Elyse and Jane will find things to say or do to Donna to make her life at work miserable, but they're things that I technically can't fire them for, just remind them to do better next time (such as writing sloppy orders or saying things that are borderline rude). Donna is extremely sweet. She's literally the perfect employee, and she treats Elyse and Jane well despite what they do to her. However, I'm stuck. Elyse and Jane were fine without Donna around. They worked well, but Jane has to be replaced anyway due to her work ethic. So, do I replace Donna or Elyse? Both are hard working, but Donna has been here for only a while, and Elyse has been working for me for over 4 years. However, in the short time I've known Donna, I feel closer to her than Elyse, and I find I work better with her. That being said, Elyse is better at asserting herself and she doesn't put up with a lot. WHAT DO I DO? Fire them both and hire again? Fire just one...?