How do i stop a older cat from bullying a younger one? ?

I have 3 cats. Ones 9, another 5, and the youngest is 3. My 5 year old cat is constantly attacking my 3 year old cat. The 3 year old is not completely defenseless, she does attack her when needed, but i don't know why the 5 year old has any reason to attack any cat! They are all females, get outside at different times, share that same meals, and get treated the same. is there any way i can stop this? My parents feel i cannot get a non caged animal because the 5 year old would attack it. Is there any solution for this?