Are there more seasons of xxxHolic? The only seasons that I can find are 1 and 2 and I think 3.
Ok, i am unsure what you mean by a 3rd Season but here's what i've looked up, there is
xxxHOLiC (Season 1) which was saw a number of releases outside Japan, 24 Episodes
xxxHOLiC: Kei (Season 2), this hasn't see as many releases, in the link there is a French Translator and Arabic Broadcaster but no licencing or distribution companies are listed, 13 Episodes
xxxHOLiC: Shunmuki (OVA), so far only dench is listed, 2 Episodes
xxxHOLiC: Rō (OVA), only in japan, 1 Episode
xxxHOLiC: Rō Adayume (OAV), once again only in japan, 1 Episode
there is also a xxxHOLiC movie called xxxHOLiC the Movie: A Midsummer Night's Dream, numerous releases and from what i've seen in Australia gets released with Tsubasa RESERVoir CHRoNiCLE the Movie: The Princess in the Birdcage Kingdom aswell.
Finally, i should forget the Manga which is released in English.
the List above is i'm assuming the chronological order as the links all seem to indicate that much but i haven't seen the series myself however xxxHOLiC ties itself with Tsubasa RESERVoir CHRoNiCLE from my understanding.
To answer you main question How many seasons are there of xxxHolic i'd say only 2 Seasons and 4 OVA's (remember Shunmuki has 2 episodes)
Now i have no idea how long the OVA episodes go for however i accept Maria Watches Over Us OVA Season 3 is a season by itself as that has 4 episodes about 40-50 minutes each, in a sense that's about 8-10 regular episodes of a normal series or if xxxHOLiC's episodes OVA episodes are about that you can refer to them as seasons aswell