inputting grades (javascript function)

How do I make it so that the numbers I input are all on top of one another and another together in one spacing.

It supposed to show all the 3 grades I have input and when the button is clicked it should show the average and either pass or fail (I know it's supposed to be an if else statement but I really can't comprehend the codes and where will I put it)

If someone helps me solve this one can someone give me more user input / function related exercises that I can work on? thanks.

var p = prompt ("enter first grade");
var c = prompt ("enter second");
var o = prompt ("enter third grade");
document.write (p);
document.write (c);
document.write (o);

function xxx (p,c,o)
{
    document.getElementById("demo4").innerHTML = ((parseInt(p)+parseInt(c)+parseInt(o)) / 3)
}

<p id="demo"></p><p id="demo2"></p><p id="demo3"></p><button onclick="xxx()">calculate</button><p id="demo4"></p><p id="demo5"></p>