Random Dice

//in the Button class
private int sides;

public Button(int sides)
{
    this.sides = sides;
}

public void act()
{
    if (Greenfoot.mouseClicked(this))
    {
        getWorld().removeObjects(getWorld().getObjects( Number.class ));
        getWorld().addObject(new Number(sides));
    }
}

//in the Number class (or however you called it)
private int sides; // in case you don't want to just display it, but maybe need it again later. So if you don't create a new object every time.

public Number(int sides)
{
    this.sides = sides;
    displayNumber(sides);
}

public void displayNumber(int sides)
{
    int s = Greenfoot.getRandomNumber(sides);
    //display the number
}

private Button buttonOne, buttonTwo, buttonThree;

// after setting button fields with the button objects, you can do the following in the act method
if (Greenfoot.mouseClicked(buttonOne)) // first dice selected
if (Greenfoot.mouseClicked(buttonTwo)) // second dice selected
if (Greenfoot.mouseClicked(buttonThree)) // third dice selected