Maybe it's easier to see the vampire ones because they've been around each other *so long* that they've tried everything at least twice.

I don't know the Vampire Diaries, but the Buffyverse vamp OT3s are totally canon in my reading. My brain is tripping over Darla/Lindsey with or without Angel, but hey, to each her own.

Angel/Cordy/Doyle and Wesley/Fred/Gunn are classic "We have (at least) sort of a love triangle. Let's make it better with a threesome!" groupings. I think they'd work better as one-offs than by trying to make things long-term (canon notwithstanding).