Please help!

I have this cat, shes about 7 or 8. She likes to pee outside the litter box. But I think shes marking her territory because of standing up while doing it. We have two cat boxes. We have a home with 2 other cats. All the cats are fixed. She has 3 main spots, the 2 bathrooms and behind my bedroom door. How do I stop this? Shes been doing this all her life