Any ideas??

Just like males, some females also can "spray" to mark territory, and the litter box is a favorite marking place... afterall, they would like to smell only themselves there, and not the other 2 cats! You may not be able to stop her from doing this, but I have another solution which might work much better. Get a larger/HIGHER litterbox. I am totally bewildered why the pet industry thinks that a litterbox can be no higher than 4" tall!!! I have used a "storage bin" for YEARS and don't have the litter-SPREAD problems and the "pee outside the box" problems others do. If cats can jump up onto countertops, a 12" tall box poses no problems for them whatsoever. Go into the department store's "housewares" department where plastic storage bins are sold. There are bins of ALL shapes and sizes, and I suggest something at LEAST 12" tall. That leaves plenty of room for lots of litter AND plenty of room for lots of "shooshing" by the cats. I had one cat, Angel, who just ADORED "shooshing" in the "poo-pit" ... and I TIMED him once... took 2 minutes of shooshing to satisfy him! But he loved it! But having a TALL box prevented the litter from being strewn all over the floor. Oh... and when you clean up the pee areas, be SURE that you use either a cleaning solution with some bleach in it (not heavy!) or purchase a commercially available urine cleaner that uses ENZYMES to clean. Reason? To TOTALLY eliminate the smell of the urine. Cats tend to pee over the same areas where they smell the pee from previous times. If you do NOT eliminate ALL traces of the smell (WE can't smell it but THEY do!), then they will continue to mark, and re-mark the areas. I had a cat who peed on the bathmat in the tub ALL the time. Finally, I had to totally wash/clean the tub with bleach, I had to TOSS the old bathmat and replace it with a new one, and FINALLY the cat stopped peeing in the tub. GOOD LUCK!