To load the data from a text file into the program, see loadStrings().
Finding the smallest and largest can then be done with min() and max().
Calculating the average of a number cannot be done with built-in Processing functions, although it isn't very complicated.

To find the average, add all of the numbers and divide the sum by the number of numbers.
I'll leave the creation of the function up to you. For reference, take a look at some of the tutorials on the main site - they help with code structuring and other basics.