Title: A Phase 2 Trial of Pembrolizumab (MK-3475) in Combination with Platinum Doublet Chemotherapy and Radiotherapy for Participants with Unresectable,Locally Advanced Stage III Non-Small Cell Lung Cancer (NSCLC) (KEYNOTE-799) Description: The purpose of this study is to see how well the study drug (pembrolizumab together with 2 different chemotherapy combinations and radiotherapy) works and to test the safety of the study drug. The chemotherapy combinations (carboplatin with paclitaxel OR cisplatin with pemetrexed) and radiotherapy being used are approved treatments for your type of non-small cell lung cancer (NSCLC). Pembrolizumab is being tested and is not approved for sale for your type of NSCLC. You have been assigned to treatment based on your type of cancer and the chemotherapy combination the study doctor thinks is best for you. Your participation in this study will last about 1 year on study drug, as long as it is safe and your cancer is not getting worse. After treatment, you will enter the follow-up period. Your study doctor will discuss how often you will need to come to the clinic or be contacted during the follow-up period.