how is cumulative gpa calculated for semesters?

This might be a dumb question but since I couldn't find what I am looking for, I decided to ask here. I think the cumulative gpa is the average of all each semster's gpa, however the part that I don't understand is if I have the below points for three semesters,

1st semester  = 3.1

2nd semester  = 3.4

3rd semester = 2.9

would the cumulative gpa for the three semesters be calculated as

  ((1st + 2nd)/2 +3rd)/2 or (1st + 2nd + 3rd)/3 ?