Katy perry's song 'last friday night (tgif)' ?

in the song 'last friday night (t.g.i.f)' by katy perry, there are a few words that i don't know the meanings of:
skinny dipping?
and menage a trois?
please don't tease me about it, i just want a straight answer, so just tell me what they all mean. thankyou :) x
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