one approach is that you store all the values entered by the user in a container, e.g. vector or a list.
After the user has finished the input you can calculate the average, min, and max.
Another way could be that you start with the lowest possible maximum and the highest possible minimum.
Then for each iteration you compare the input with min and max and change these values if appropriate.
Also sum up all inputs in a variable that you can than use to calculate the average (suppose you count the loop iterations).