You will almost certainly need surgery to treat stage 3 rectal cancer. Your doctor may recommend chemotherapy and radiation therapy first to shrink the cancer followed by surgery to remove the part of the rectum that contains the cancer. Nearby lymph nodes will also be removed. Often, the healthy parts of the colon can be fused together again, but depending on the exact location of rectal cancer, a colostomy may be required.
Since lymph nodes are affected by cancer at stage 3, you will need additional treatment after surgery to prevent the cancer from spreading. Your doctor will likely recommend a combination of chemotherapy and radiation to continue treatment, and continued vigilance to catch the cancer if it returns.