How to unpivot 3 headers with a two sidebar

You want to transpose, fill down, add a column for pivoting and put it at the front.
Remove COlumn1 and 2, transpose back, promote header and unpivot other column.
you can now split your pivoted column back to their original values
Sample code is below as a demo 

Not the simplest transform, but it gets the job done.

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Transposed Table" = Table.Transpose(Source),
    #"Filled Down" = Table.FillDown(#"Transposed Table",{"Column1"}),
    AddPivotHeader = Table.AddColumn(#"Filled Down", "PivotHeader", each [Column1] & "-" & [Column2]),
    #"Removed Columns" = Table.SelectColumns(AddPivotHeader,
{"PivotHeader"} & List.Select( Table.ColumnNames(AddPivotHeader), 
each not List.Contains({"Column1","Column2","PivotHeader"}, _))),
    #"Transposed Table1" = Table.Transpose(#"Removed Columns"),
    #"Promoted Headers" = Table.PromoteHeaders(#"Transposed Table1", [PromoteAllScalars=true]),
    #"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Promoted Headers", {"Column1"}, "Attribute", "Value"),
    #"Split Column by Delimiter" = Table.SplitColumn(#"Unpivoted Other Columns", "Attribute", Splitter.SplitTextByDelimiter("-", QuoteStyle.Csv), {"FirstRow", "SecondRow"})
in
    #"Split Column by Delimiter"

Refer

https://radacad.com/pivot-and-unpivot-with-power-bi

Appreciate your Kudos. In case, this is the solution you are looking for, mark it as the Solution.
Thanks.

My Recent Blog - https://community.powerbi.com/t5/Community-Blog/Comparing-Data-Across-Date-Ranges/ba-p/823601