Have you ever tried using Feliway
? It's a synthetic version of the "happy" cheek pheromones cats use to rub on stuff to remind them of good times. Can help relieve anxiety in some cats. I'd recommend trying a couple of diffusers in areas of your home where the cats commonly hang out.
I wonder if something happened (like a loud noise or something), that scared Checkers while she was in the presence of one of your other cats and she now associates her with that scary incident. Some things you could do to help re-establish a better relationship between them is to create positive associations while they're together. Give them their favourite treats or some tasty morsels of canned low sodium salmon or sardines when they're at a distance that doesn't cause Checkers too much stress. Slowly move the treats closer. Also play therapy can be useful. Have someone play with one cat while you play with another, using a wand or fishing rod type of toy, until Checkers forgets what she's mad about.