h1 { counter-increment: h1; counter-reset: h2 h3 }
h2 { counter-increment: h2; counter-reset: h3 }
h3 { counter-increment: h3; }

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