If this is going to happen, Faith's got some thoughts on the matter.
Couple different ways this could go, almost all of which she's less than not interested in. Put Buffy in the middle, on her back, pretty golden princess bookended by the two dark-haired striving, yearning, lovers, petting her, worshiping.
Hold Buffy between her legs, kissing her neck, cupping her little budlike breasts, bruising rosy nipples, while Angel drives into her, hard enough to rock Faith back against the wall, skillful enough to make Buffy twist-cry-come without touching her clit, like a good woman, constructed for him.
Let the two girls climb over his body, trail twinned small hands down his chest, wrap around his cock, kiss him with doubled tongues, touch and lips and breath urging him to come.
Nor should they kneel together, dark and light heads bowed, working red lips over his cock while he lies back on his elbows, admiring and savoring.
She's not going to be on the outside any more, third wheel and second fiddle. If this is going to happen, she's going to be right there and it's not going to be their pure true love plus the dirty dark twin, not going to be two hungry women who quiver at his crooked finger and obey his groaning needs.
It's not going to be 2+1 any longer. It's going to be messy and chaotic, three all at once, it's going to be her tongue spiralling inside Buffy while Angel clutches her own ass, raking his nails and slapping her red, it's going to be Buffy's fingers in her cunt and Angel's dick down her throat, it's going to be sweat and come sliding them around, skin sticking and parting, and it's going to be hot and dark as blood and candlewax, embers flaring high and pain brightening, contorting, into pleasure.
It's going to be great.
If she gets her way.
Which she fully intends to.