stairs

  1. Go downstairs.
  2. Get off upstairs.
  3. Get down from upstairs.
    Which are correct?
    What is the right native expression for this?

Go downstairs/go upstairs <-- standard commands (‘Go Downstairs’ - you are upstairs, the listener is upstairs)
Come downstairs / Come upstairs <-- standard commands (‘Come downstairs’ - you are downstairs, the listener is upstairs)
Get off upstairs / get off downstairs <-- Not the usual term. Used in a particular exclamatory context only. Used in a similar manner to ‘Be off with you!’ May also be heard as ‘Get yourself off upstairs.’
Get down from upstairs <-- Not the usual term. Used in a particular exclamatory context only. "Get up from downstairs’ is an even more unusual phrase.
It is very difficult to describe in words the contexts in which your versions 2 and 3 are used.