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!
by kofi June 30, 2003
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