<style>#one, #two, #three {display: none}</style>
<script>
document.write(
  '<style>#', 
  ['one', 'two', 'three'][Math.floor(Math.random()*3)],
  '{display: block}</style>'
);
</script>

<style>
#a1, #a2, #a3 {
	display: none;
}
</style>
<div id="a1">a1</div>
<div id="a2">a2</div>
<div id="a3">a3</div>

<script>
document.addEventListener("DOMContentLoaded", () => { 
  let rnd = ["a1", "a2", "a3"][Math.floor(Math.random()*3)];
  document.getElementById(rnd).style.display = 'block';
});
</script>