<a href="#" onclick="show('bir')">1</a>
<a href="#" onclick="show('iki')">2</a>
<a href="#" onclick="show('uc')">3</a>

<div id="bir" class="test">
<p>
Div 1
</p>
</div>
<div id="iki" class="test" style="display:none">
<p>
Div 2
</p>
</div>
<div id="uc" class="test" style="display:none">
<p>
Div 3
</p>
</div>

<script>
function show(id){
document.querySelectorAll('.test').forEach(function(el) {
   el.style.display = 'none';
});
document.getElementById(id).style.display= 'block';
}
</script>