I typically only use ternaries for single operations and extract to a function if it's too big. Although they are quite fun in JSX. For your code i'd probably do:

  function SetPosition(x, y, z) {
    if (!(isNumber(x) && isNumber(y) && isNumber(z))) {
      // Default vals
      return;
    }
    x = clamp(x, 0, 1337);
    y = clamp(y, 0, 1337);
    z = z;
  }