A
threesome as a
one-off (so to speak). A three-in-a-bed is a sexual encounter between three
people, usually (at least for two of those involved) for the first and last time.
A phrase beloved of the British tabloid
press is "three-in-a-bed
romp", which stresses
bother the impermanence and the (just-for-)fun nature of the experience, thus engaging the sexual interest and desire, the envy and thus the moralising indignation of the newspaper's readers all at once!