<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>