Ditto to what everyone here has said. A trip to the vet is in order to rule out any medical problems like UTI. Rule of thumb - one litterbox per cat.
You need to get the last cat spayed before she goes into heat.
Question: Have you changed the cat litter lately? Cats don't like change. I know Bad A$$ Buddy will pee in the bathroom (tile floor thank goodness) on the floor next to the box (I have a plastic dish tray under to catch litter) if his box is not to his liking (I have 5 cats and they all seem to gravitate to the bathroom box, even though I keep 3 Rubbermaid tubs as litterboxes in another room). I know it's a hint that he's not happy with it. This morning he peed in the box I had cans of Fancy Feast in cuz the bathroom box was reduced to sawdust (I use Feline Pine).
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