No, they don't mean the same, and none of them is really a general purpose expression for "move a bit further". In the sense that you're asking about, these are the usual meanings: "back up" = reverse a vehicle "back off" = move away from someone or something, especially in order to avoid danger or a confrontation "back down" does not usually refer to physical movement. It means to abandon or soften one's position in an argument or disagreement, and concede to or become more conciliatory towards the other party.
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tinanam0102Thank you very much for the examples. Is "over" in "move over a bit" optional?Grammatically it's optional. Idiomatically, in the kind of situations we're discussing, "over" (or some similar word) helps to smooth and soften the request, so is advisable.