My mom just told me that she has stage 3 cervical cancer. i'm worried if this is life threatening?

Yes: Stage 3 means that the cancer has spread to the lower vagina (3a) or the side of the pelvis (3b). This can certainly be life threatening. The usual treatment is radiation and chemotherapy at the same time with relatively good success. Her doctor may or may not do lymph node staging before radiation to evaluate how far up to give the radiation. She does need treatment right away.