of the ones I've seen or know something about:

Spike, Angel, Xander
shag spike as I bet he'd be fabulous. Cliff Angel as he'd just mope around with furrowed brow, and marry Xander as I bet he cooks real good.

Lana Lang, Lois Lane, Chloe Sullivan
marry Chloe as she's v.v.hott and I'd like to spend as long as possible gazing at her. Shag Lana as she's v.hott and cliff the other one by default.

The Devil, from Bedazzled (Elizabeth Hurley), Christina Ricci, Drew Barrymore
Hmm. trickier one, this. I'm not sure I could bring myself to shag Drew, much as I'd like too as it'd feel wrong as she was cute moppet from ET. That said, she's now teh hott. Argh. Cliff Ricci though. OK, shag Liz (rawr) and marry Drew.

The Devil from the original Bedazzled, Peter Cook, The Devil from Mr. Frost, Jeff Goldblum, The Devil from Angel Heart, Robert De Niro
Cliff De Niro as he's scary when cross. Shag Jeff as I bet he'd be fabulous and marry Peter Cook, despite him being Dead.

From Torchwood: Jack, Owen and Ianto
Shag (repeatedly) then marry, cliff, cliff.

:-)

ok. Cliff Owen and marry Ianto. He also looks like he'd be useful around the house, when he's not off pouting.