I'm actually moving out Melba for Cheri cx
I already have Bunnie and i love her. But among your three, I'd keep Cheri.
Im not a fan of the cats and dogs in ACNL c:
In my opinion, you should keep Cookie. I used to have Cookie in my town, but I lost her a couple months ago. She originally came and smashed some of my hybrids, and plotted right next to my house, but after awhile she grew on me, and I'm finally starting to miss her.
I'd choose either Cheri or Merry, mostly because I'm not a personal fan of Cookie, but honestly, you should choose whoever you feel closest to in game!