Multiple ifs javascript, don't working

Why nothing happens? I would like to make a script giving the highest value from 3 variables.

var a = parseInt(prompt("Variable giving", ""));
var b = parseInt(prompt("Variable giving", ""));
var c = parseInt(prompt("Variable giving", ""));
var m = 2;
var m = a;

if (b > m)
{
  m = b;
}

if (c > m)
{
  m = c;
}

if (d > m)
{
  m = d;
}

if (a != c || a != b || b != c)
{
  document.write(m);
}
else
{
  document.write("One of variables is as same as another one");
}