Hi thanks for your question
Since the cancer is limited to lung only and has not spread beyond lung,If the cancer is limited to one lung only pneumonectomy(removal of whole lung) is a possibility,otherwise Chemo/Radiotherapy is treatment of choice depending upon the type of cancer he is having.He can live long with the appropriate treatment.There are powerful analgesic(pain killer) which can render his life pain free till he lives.
Hope this answers your question

Hello,
Sorry to learn about your uncle's condition.
While there are several treatment options available, the prognosis is generally poor.
You must get him the proper treatment and forget about the timing and other things pertaining to death.
I have two things to tell you.

1st.... While stage 3 lung cancer may or may not be curable, it is treatable. These treatments may not only improve survival, but help with the symptoms of lung cancer as well. Several treatments are currently being evaluated in clinical trials, and offer hope that stage 3 lung cancer life expectancy will improve in the future. The National Cancer Institute states that all individuals with stage 3 lung cancer should be considered candidates for clinical trials.

2nd...If docs are providing treatment ,then they at least have hope of extending life and quality of life, for your uncle.
I  would advise that since that time cannot be precisely measured even in the worst case scenario, that it be lived as if every day were the last, spent enjoying life, friends, family, spent loving and laughing. 
Keep giving her  good treatment and good psychological support and care and forget about the timing of death,which nobody can tell. 
Best of luck
Thanks