function ThreePics() {
  a = Math.floor(Math.random() * 12) + 1;
  b = Math.floor(Math.random() * 12) + 1;
  c = Math.floor(Math.random() * 12) + 1;

  if((a != b) && (a != c) && (b != c)) {
    document.images["b1"].src = "bilder/" + a + ".jpg";
    document.images["b2"].src = "bilder/" + b + ".jpg";
    document.images["b3"].src = "bilder/" + c + ".jpg";
  } else ThreePics();
}