I distinctly remember there being one that has Sirius, Remus, and Snape as a threesome couple. I remember reading this a long time ago and I really enjoyed it then. The plot goes something like this: (Really simplified)

The younger versions of the above-mentioned characters accidentally travel to the future, where they meet their older counterparts, their presense aiding somehow in the war or something like that. In the end, they go back to their own time happily together and in love. Anyway, all I remember are the pairings that come out in the end. Here they are: (I don't know if there's an order for them.)

(Young!) Sirius/Remus/Severus
Sirius/Remus/Severus

Harry/Draco (?... or did this become a threesome, too?)

Thanks, in advance, for anyone who helps!