If you have a "good" polarizer, its perfectly fine to stack it with a good, high-quality ND filter. About your only problem would be possible vignetting from stacking the filters, and how much vignetting you get will depend on your focal length.
I doubt that a 3-stop ND by itself would be enough under those conditions (1pm on a bright sunny day). I assume you're thinking about waterfall shots or something similar, such as smoothing out water? For example, let's assume the following: your camera meter reads f/11 at 1/500 sec, and you're set to ISO 100. A 3-stop ND will slow the shutter speed down to about 1/60 sec...not enough to blur water or smooth the surface. Adding in a polarizer can give you up to another two stops of light reduction...this would bring it down to about 1/15 sec. You'd need to get down to about 1/15 sec at the bare minimum, preferably down to about 1/5 sec where you'll see the silky effects on the water. You can go to a smaller aperture, but then you'd need to consider lens diffraction coming into play. But a lot depends on the conditions at the time. If your meter was reading f/8 instead of f/11, you'd have an extra stop or two of reduction available, which would work for you. Like someone else has suggested, a 6-stop ND filter would be your best choice.
Not sure what size filter you're looking for, but here's a link to a B+W 6-stop filter:
http://hvstar.net …Action=VIEWPROD&ProdID=54
EDIT: Sorry...just noticed the "10 second" shutter speed in the original post. You can use my example to calculate how many stops you'd need to get down to 10 seconds, but definitely, 6-stops will be nowhere enough under your conditions. You'd need an ND filter somewhere in the neighborhood of 10-stops, plus the polarizer.
So i guess i'll need a ND filter; but I even think that a 3stop nd (hoya 8x) won't be enough.
Thanks again.
