Combining multiple columns in a single table into one long column

If you don't need the null values, then you can unpivot all columns:

let
    Source = Input,
    #"Unpivoted Only Selected Columns" = Table.Unpivot(Source, {"Column1", "Column2", "Column3"}, "Attribute", "Conference Type"),
    #"Removed Columns" = Table.RemoveColumns(#"Unpivoted Only Selected Columns",{"Attribute"})
in
    #"Removed Columns"

If you want to keep the null values, then you can convert the table into columns (lists), combine these lists and convert the result back to a table:

let
    Source = Input,
    Result = Table.FromColumns({List.Combine(Table.ToColumns(Source))},type table[#"Conference Type" = text])
in
    Result