, (max z-index), onmouseover.
, div2 div z-, div2 z-, .
div3 , div2, onmouseover, div2.
<body>
<div id="div1" style="position: absolute; z-index: 10; left:0; top; 0; width: 100%; height: 100%; opacity: 0.25; background-color: black;"></div>
<div id="div3" style="z-index: 20; position: relative;">
<div id="div2" style="z-index: 0; position: relative;">
</div>
</div>
</body>