The exact treatment and outcome differs according to site and type of tumour. For example, stage 3 breast cancer of one type would differ from other types of breast cancer. Similarly they are different from stage 3 stomach cancer.
Having said that, some general principles are as follows. Most stage 3 cancers are considered as inoperable. With exceptions of head and neck tumours. They can treated with either chemotherapy or radiotherapy or both to shrink them and consider operation. When operation is not possible radiotherapy along with chemotherapy are good options.
The cure rates of stage 3 vary from 30 to 60 %.