Does 3rd stage in lungs cancer curable or can we prolong patient's life by treatment ..?

No: No but life can be prolonged with chemo and radiation. Each patient response to treatment will be different and also depend on the tumor type.

Sometimes: There are some long-term remissions with stage 3 lung cancer ranging from 10 to 20% depending on the extent of lymph nodes with cancer in the mediastinum. Concurrent chemotherapy and radiation +/- surgery can prolong substantially patient's life.