Someone who spends a lot of time in bed; this is not because they’re ill, but because they’re a bone idle, useless, twat who is totally unfit for purpose as part of the human race.
“Malcolm is always late for work; he says he’s got chronic fatigue syndrome.”
“Nah, he’s just a uselessslug-a-bed cunt.”
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!