How is cumulative GPA calculated?
The cumulative GPA can't in general be known from just the GPAs of the individual semesters.  The cumulative GPA is your total grade points divided by your total possible grade points, over all semesters completed so far.  If you took the same number of classes each semester, and all had equal numbers of units, and all were included in GPA (i.e., no pass/fail or the like), then it would be (1st+2nd+3rd)/3.

The reason it can't be known just from the GPA of the individual semesters is that it depends on how many units you took each semester.  If you took more classes/units in one semester than in another, then that semester will have greater weight in the cumulative GPA.

This applies to averages in general.  If I tell you I have one bag of stones with an average weight of 1kg, and another bag of stones with an average weight of 2kg, you don't know what the average weight would be if you combined all the stones into one group.  To know that, you have to know how many stones are in each bag.