Except that TW3 is a lot gorier than Buffy, and horror in Buffy was always lifted by some humour - except for a few episodes that were downright serious and troubling ("The Body", anyone? Gets me everytime... can't watch it, too sad). But even horror episodes like the one involving the Gentlemen - for me the creepiest monsters on Buffy ever, throughout all 7 seasons - have some humour in them to uplift the tone. Still, I can see why you draw that comparison.
The Oxenfurt Drunk comes to mind as a Buffy-like quest, where Geralt calls the vampire a bitch. It had me laughing so much.
I think even the monsters themselves in TW3 have humour, in the English version nearly everybody including the Crones sound like British farmers from the west country and even the Grave Hags have that accent. Watch Hot Fuzz and you'll see what I mean, it's hard to take that accent seriously. Along with that, monsters interact with the world in a weird way, maybe it's the whole thing to do with the Conjunction of the Spheres but I've never felt like the atmosphere and the monsters have any symbiosis.
A lot of the monsters that only came out during the night in TW1 have a 24/7 appearance in TW3. I remember having to meditate because the drowners would only come out at a night, which made sense. Speaking of which, some of the more humanoid monsters felt like they were wearing prosthetics. The Crones for instance didn't come off as repugnant magical beings but rather disproportionate people wearing cosplay and the botchling quest's horror was shut down completely by the fact that when it turned into a lubberkin (which is a funny sounding word) and becomes a floating ghost fetus which just seems ridiculous.
One things for sure is that in B&W I was looking for Manticore gear and there was a bruxa in human form just wondering around and it made me jump when I tried getting closer, one of the only moments in the game that made me do that.
Maybe I'm just too accustomed to things most people find scary. But TW3 has little in terms of horror devices, a core strength to horror in general is the feeling of not having any control but Geralt is always in control.