I had a total knee replacement three years ago i have done two marathons and walk and swim every day. i'm having significant pain. is this normal?

Not normal : There are people with new knee replacements who do complete triathlons successfully. Although a person may have knee pain (even without a joint replacement) for a few days after a marathon I would not expect you to have severe pain for weeks after an event. Occasionally the hardware of the new knee replacement can loosen resulting in pain symptoms. Please follow up with your orthopedic doctor.

Need a xray & exam: I have patients with knee replacements that run marathons also. I am always concerned for them. This high impact activity can cause several concerns. The inserts can wear faster, the implants can loosen from the bone cement connection, the bone can develop stress fractures under the implants, scar tissue can build up causing pain requiring arthroscopic clean out, tendons can get inflamed.