How do I train my cat to start using a litter box again?

I recently moved into my girlfriends house with my female cat, Guinness. My girlfriend already had two female cats living with her. Guinness is the oldest and biggest out of the three and still after months does not get along with her cats (fighting, hissing, etc... at every chance). The most stressfull thing is she refuses to use any of the three litter boxes I have set up for them. I clean them out daily and even tried to strategically place them throughout the basement for encouragement.... no use. I still come home everyday to piles on the floor around the litter boxes.