Option 1 Each div is specifically identified, so any other div (without the specific IDs) on the page will not obey the :hover pseudo-class.
<style type="text/css">
#div1, #div2, #div3{
display:none;
}
#div1:hover, #div2:hover, #div3:hover{
display:block;
}
</style>
Option 2 All divs on the page, regardless of IDs, have the hover effect.
<style type="text/css">
div{
display:none;
}
div:hover{
display:block;
}
</style>