You can't add more than one filter property but you can add more than one filter value.
So code would look like this:
progid:DXImageTransform.Microsoft.Blur(PixelRadius=3, MakeShadow=true, ShadowOpacity=0.30);
However the blur filter will just blur the gradient and can't be used in combination with the gradient filter (as far as I can tell). Remove the blur filter and your gradient will show.
I belive that to get the blur (shadow) effect you would need to add the blur to an absolutely place div which is shimmed under the gradient so that the filters are not in the same stacking content.