body {
  counter-reset: h1;
}

h1 {
  counter-reset: h2;
}

h1::before {
  counter-increment: h1;
  content: counter(h1) ". ";
}

h2::before {
  counter-increment: h2;
  content: counter(h1) "." counter(h2) " ";
}

h2 {
  counter-reset: h3;
}

h3::before {
  counter-increment: h3;
  content: counter(h1) "." counter(h2) "." counter(h3) " ";
}