the show_result function
function show_result() {
  for (var i=0;i<g_result.length;i++) {
    var ids=g_result[i];
    for (var j=0;j<3;j++) {
      var id=ids[j];
      var el=document.getElementById(id);
      el.style.backgroundColor='pink';
    }
  }
}
implementing check & show_result
function square_clicked(el) {
  el.textContent='X';
  var id=el.id;
  g_squares[id]=1;
  if (check()) show_result();
}

You can now test that it all works. You should be able to end up with every square pink!