3 doesn't seem like too many to the one who has 4...In our defense, our 4th was entirely our DS's fault. He was 6 and riding around the driveway on his bike and this adorable little gray beast kept chasing him back-and-forth. So we took him in and named him Chase. We have no regrets.
Another thought...are your boxes covered? Some cats like covered boxes, some prefer open. Perhaps get a new box that is a different style than what you have. We have two MASSIVE boxes. They are like 18" x 24" and really deep. The funny thing we have in our house is that our cats seem to use one for #1 and the other for #2. Makes me wonder which one of them made that rule.