Painful left lump on left side of rib cage. It has grown in size. MRI was last test to show something but it was not defined. Been 3 yrs, what is it?

Need more info. If the lump is in the rib itself, did you have trauma in that area? Have you ever had a cancer, especially of a type that tends to spread to bone; and does the MRI suggest a cancer in the bone? That would be my concern, although your description makes me doubt cancer. Review the findings with your clinician. A biopsy, if needed, is one way to answer your question. Hope that helps.
Hard to say. There are certainly many possibilities. If there is any concern or the diagnosis is in question, an excisional biopsy would probably be reasonable. Don't hesitate to discuss with your doctor.
Consider a new scan. It is possible that 3 years ago the abnormality was too small to adequately characterize. But it has grown since then and it's still painful - you owe it to yourself to find out what it is. A new CT may be a reasonable first step then moving to MR if a definite abnormality is observed. Review the old study with a radiologist and discuss with your primary doc. Time to get an answer!