Buffy would have to be horrendously retconned & OOC to ever share one of her men with another woman.
I can see Angel & Spike going through the love triangle with Buffy for so long that they think they can handle being in a triad relationship but then having a threesome and one of the guys breaks up with her afterwards because it's one thing to know she's done those things with HIM but it's another to be in the room while it's happening. And then Buffy would feel yet more shame and confliction about her sex life.
So I say Buffy lives happily ever after with Angel while Drusilla gets a soul and therapy and Spike lives a happy enough life with her, missing Buffy but okay knowing she has what she deserves (and what she wanted s1-6 but the damn writers thought nope, let's give Spike instead of Angel an anchored soul since she told Spike to move on but he wants her to love him like she loves Angel and that trumps in s6 Buffy thinking of Angel while Anya talks about the universe allowing her to be with her soulmate and have everything work out and in s5 Buffy saying she wants Angel to stay forever and in s4 when she had her future with Angel erased after only 24 hours as she cried, "I'll never forget. I'll never forget.").