<script>
function calcA(x,y,z){return x.value * y.value * z.value;}
</script>
<form onsubmit="return false" oninput="a.value = calcA(x,y,z)">
Объем:
<input type="number" value="1" name="x"/>*
<input type="number" value="1" name="y"/>*
<input type="number" value="1" name="z"/>=
<output name="a"/>
</form>