Multiple years Financial forecasts: differences?

I would use power query to unpivot the "Budget Year", "Budget Year + 1" & "Budget Year +2" columns, then rename them to [Year], [Year] + 1, [Year] +2 respectively and convert to whole numbers.  Then use a dax calculated column or measure to compute the numbers you want.

Here is Power Query I generated:

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Year", Int64.Type}, {"Account", Int64.Type}, {"Budget Year", type number}, {"Budget Year + 1", type number}, {"Budget Year + 2", type number}}),
    #"Unpivoted Columns" = Table.UnpivotOtherColumns(#"Changed Type", {"Year", "Account"}, "Attribute", "Value"),
    #"Added Conditional Column1" = Table.AddColumn(#"Unpivoted Columns", "Custom", each if [Attribute] = "Budget Year" then [Year] else if [Attribute] = "Budget Year + 1" then [Year] + 1 else [Year] + 2 ),
    #"Renamed Columns" = Table.RenameColumns(#"Added Conditional Column1",{{"Custom", "Budget Year"}}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Renamed Columns",{{"Budget Year", Int64.Type}}),
    #"Removed Columns1" = Table.RemoveColumns(#"Changed Type1",{"Attribute"}),
    #"Reordered Columns" = Table.ReorderColumns(#"Removed Columns1",{"Year", "Budget Year", "Account", "Value"})
in
    #"Reordered Columns"

then a dax calculated column:

=
VAR this_year_value = Table1[Value]
VAR this_year_budget_year = Table1[Budget Year]
VAR Next_year = Table1[Year] + 1
VAR next_year_budget =
    CALCULATE (
        MAX ( Table1[Value] ),
        FILTER (
            ALLEXCEPT ( Table1, Table1[Account] ),
            Table1[Year] = Next_year
                && Table1[Budget Year] = this_year_budget_year
        )
    )
RETURN
    this_year_value - next_year_budget

I'm sure you could do as a measure if needed, but i'll leave that to you.  Good luck.