You could wrap the hidden div in another div that will toggle the visibility with onMouseOver and onMouseOut event handlers in JavaScript:
<style type="text/css">
#div1, #div2, #div3 {
visibility: hidden;
}
</style>
<script>
function show(id) {
document.getElementById(id).style.visibility = "visible";
}
function hide(id) {
document.getElementById(id).style.visibility = "hidden";
}
</script>
<div onMouseOver="show('div1')" onMouseOut="hide('div1')">
<div id="div1">Div 1 Content</div>
</div>
<div onMouseOver="show('div2')" onMouseOut="hide('div2')">
<div id="div2">Div 2 Content</div>
</div>
<div onMouseOver="show('div3')" onMouseOut="hide('div3')">
<div id="div3">Div 3 Content</div>
</div>