take your kitty to the vet to rule out any medical problem that might be causing the inappropriate elimination.
you should have one litter box per cat in the house, plus one extra litter box, as a general rule. some cats have substrate preferences, so you could try regular litter in one, clumping in another, feline pine in a third, etc, and see which one kitty goes for. most cats also like to go in a clean box (can’t say i blame them, i would rather use nordstrom’s bathroom than a truck stop’s), so make sure you clean them every day.
we recommend “kitty jail” to our clients who are having this problem. kitty stays in a large dog crate (or a bathroom) with a litter box, food and water and a place to sleep for at least a week, or until kitty is consistently using the box. gradually begin to increase the “living space” using gates, doors, or other barricades, all the time making sure kitty is only using the box. most cats will get the hang of it quickly.
you can lay down tinfoil over the spot where the cat seems to be going most frequently as a temporary fix - most cats don’t like to step on tinfoil. lastly, examine your environment closely - did one of the other cats corner this cat in the box at some point - does she get picked on by any of the other cats? are there new stray cats in the neighborhood? you can get a product called “feliway” from your vet’s that can help - it soothes stressed out kitties, and comes in a spray, and in a plug-in room diffuser.
i hope everything works out with your cat.