Small hard lump on rib x 1.5 yrs. sometimes painful. xrays x2 negative. 3 docs say its cartilidge overgrowth. still worried. should they biopsy this?

Same size?: If it has not changed in size in a year and a half, then it's pretty unlikely to be cancer. Feel free to get a second opinion though, if it will make you feel better. This opinion does not count much, since this is a general statement and I don't know your history or have access to your records. Get a second opinion with a doctor who examines you and has your past records before doing so.

Follow up with doc: If the lump has not changed much in 1.5 yrs, and it has been seen by 3 doctors and you have had 2 negative X-rays, the lump sounds benign. However, you sound worried and I would recommend that you contact the physician who saw your lump most recently, or the physician whom you trust most, and talk to him/her about further testing, which may include biopsy if appropriate.