using a method in java

I don't know if I would have gone with objects for such a simple task. however, since you asked, I'll explain how I would do it if objects are necessary,

I would create an object (class) which has an ArrayList<Integer>, this way you could calculate the average of more than just 3 numbers.

this object would have 2 public methods. void addNumber(int number) and double/int getAverage() addNumber would simply add a number to the arraylist, and getAverage would traverse over the entire list, sum up all the numbers, and divide it by its size (don't forget -1)

In the main, you create a new object of that class, then each scan of the scanner, inserts the inputed number to the array list using the addNumber method.

I think that how I would do it, given that I was instructed to use objects.

Good luck!

as a very basic example, in one .java file you can create a class that "knows how" to get input and create averages, something like:

// takes input and stores in a list which it 
// can do mathematical operations on
class InputMath
{
    // ivar to store input
    private List<int> list;

    // contructor
    public AverageInput() {}

    // gets user input and pushes to `list`
    public void getInput(String question) {}

    // works out the average of `list`
    public int average() {}

    // works out the total of `list`
    public int total() {}

    ...
}

and then you can use the class where you want, like in your 'main' function

import InputMath;

class Main
{
    public static void main(String args)
    {
        InputMath im = new InputMath();

        for(int i=0; i<3; i++)
        {
            im.getInput("Enter number " + (i+1));
        }
        System.out.println(im.average());
    }
}

but like many of the comments have suggested you really need to read through some material/references/examples and 'play around'

This is a brief example on how to make your program using a very simple class.

You could easyly change it and ask how many numbers you want to calculate the average.

/* using default package */

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

/**
 * The Class Average3.
 */
public class Average3 {

    /** The number list. */
    private List<Integer> numberList;

    /** The sum. */
    private float sum;

    /**
     * Instantiates a new my first class.
     */
    public Average3() {
        this.numberList = new ArrayList<Integer>();
        this.sum = 0;
    }

    /**
     * Adds the integer.
     *
     * @param integer the integer
     */
    public void addInteger(int integer) {
        this.numberList.add(integer);
        this.sum += integer;
    }

    /**
     * Gets the average.
     *
     * @return the average
     */
    public float getAverage() {
        return (this.sum/this.numberList.size());
    }

    /**
     *  Prints the Average.
     */
    public void printAverage() {
        System.out.print("The average of ");
        for(int j = 0; j < this.numberList.size()-1; j++) {
            Integer i = this.numberList.get(j);
            System.out.print(i.toString() + ", ");
        }

        System.out.print("and " + numberList.get(numberList.size()-1));
        System.out.println(" = " + this.getAverage());
    }

    /**
     * The main method.
     *
     * @param args the arguments
     */
    public static void main(String[] args) {
        Average3 myClass = new Average3();

        System.out.println("Enter the first integer.");
        Scanner keyboard = new Scanner(System.in);

        myClass.addInteger(keyboard.nextInt());

        System.out.println("Enter the second integer.");
        myClass.addInteger(keyboard.nextInt());

        System.out.println("Enter the third integer.");
        myClass.addInteger(keyboard.nextInt());

        myClass.printAverage();
    }
}