I am hoping someone can answer this...my grandmother just had her bladder removed due to cancer and while they were doing surgery they found more in her lymph nodes.
Squamous cells are present in the tissue of skin, respiratory and digestive tract and in the hollow organs of the body. This cancer can start in these areas and can spread to other parts of body. As you have given the history in question, it started on scalp, the in mouth and now in lymph node. Once it is reached in lymph node, it can traveled to other parts of the body. So the prognosis of stage 3 of squamous cell cancer is very poor. But time frame of life requires more information about the current conditions.
This all depends on the treatments involved and the particular patient. Everyone responds to treatments differently.