Blunt force trauma to upper chest, three years on, there is still a lump. why is it still there?

Chest lump: Depending on the actual location on your chest this could be a bone callus or a dislocation of a sternocostal joint. Regardless, it is likely of no consequence and should be nothing to worry about. If it were to become symptomatic or enlarging, then a evaluation and imaging may be necessary.

Depends : It could be a hematoma that organized and is now a fibrous mass. Could be a healed rib fracture with a large callus (new bone that's formed to heal the fracture). See your doctor. He or she should start with a chest x-ray, then perhaps a ct scan. If there's anything abnormal, see a thoracic surgeon.