to have the average in the roster

Hello to all. I am asking for help
in my roster I have a variable ‘‘height of the child’’ and another ‘‘weight of the child’’.
These measurements will be taken three times.
Now I would like to know how to get the average (of the three measurements of “child’s height”) as well as for the “child’s weight”.
Is it possible to calculate this directly when the interviewer finishes filling in the measurements?