I agree. And it's not just Buffy s3, also Angel s3...lol, no idea what's up with all the greast seasons3.
Well, first off, this season had all the best characters (even if a few of them only appeared as guest stars for an episode or so): out of my favourites we had Cordy, Angel, Faith, Buffy, Anya, Spike and even a bit of Wesley(my other 2 favourites would have been weird to see in this season, considering Darla was still dead and Lilah was in LA and nobody knew anything about W&H yet).
And other than that, great storyline. Seeing a slayer gone bad, one idiotic ridiculous watcher, Bangel going down the hills (sorry 😛), and, although it's not really a storyarc, lots and lots of Cordeliaism.
This may only apply to me, but I could also really relate to this season because it's my last year of school. And especially when I watch the last episodes now, when I see school ending for the Scoobies, it completely reminds me of me and my friends right now.