Welcome to the HW forum. I don't know anything about knee replacements, but I can tell you from my lumbar spine fusion surgery that everything took longer than I was led to believe and than I expected. Actually, it's been almost a year, and my surgery never helped my pain. But about 3 months is still very early when it comes to surgery, so if you doctor and PT are telling you it's normal, it probably is. Like Lindaloo said, be sure to follow all your doctor's and PT's instructions, including exercises your PT gives you as well as limits you're given on activities. I know it's frustrating, but hang in there.
I'd also suggest trying to search for some forums that deal primarily with knee replacements and see what you can learn from others who have gone through it.
Glad you decided to join us!