Stage 3c uterine cancer is a subset of stage 3 uterine cancer. The stages start with zero, which is just abnormal cell growth called hyperplasia and doesn’t really constitute a tumor. Stage 1 uterine cancer is confined just to the uterus. Stage 2 has spread to the cervix, and stage 3 has spread to other areas of the pelvic region, but not yet the rectum or bladder.
It’s a little harder to find specifics on what the letters a-b-c mean in the staging, but I read that stage 3c uterine cancer means that it has spread to the lymph nodes in the pelvic region.