Need column in respective arrangement

In your Transform Sample file query, you can just click "Use Headers as First row (demote headers), then click Transpose, then click "Use First Row as Headers".  Now you have:

Now when you combine your table columnns, you'll have:

Now you can just click "Use Headers as First Row", click Transpose, then click "Use First Row as Headers", and you are done:

--Nate

Based on your description, I created data to reproduce your scenario. The pbix file is attached in the end.

You need to use the merge command instead of appending, If you have used Merge, what was the problem?

Sohaib before you combine your tables, unpivot all columns but "Site" (Table.UnpivotOtherColumns), then combine (Table.Combine) and finally pivot column with attributes (all your headers are in that column after unpivot operation)

Here is an example of combining three tables with one similar column and then getting ride of null values.
Starting with...

and end up with...

Paste the code into the advanced editor of a blank query and you can review the steps...

let
    tableOneSource = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcjbUUQgyVNJRCgEynI3AnFgdqLgRsrgRQtwYWdxYKTYWAA==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [tableOneColumnOne = _t, tableOneColumnTwo = _t]),
    tableTwoSource = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcjbUUQgyVNJRCjHSUXA2AnNidaDiRsjiRghxY2RxY6XYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [tableTwoColumnOne = _t, tableTwoColumnTwo = _t]),
    tableThreeSource = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WcjbUUQgyVNJRCjHWUXA2AnNidaDiRsjiRghxY2RxY6XYWAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [tableThreeColumnOne = _t, tableThreeColumnTwo = _t]),
    combineNoRename = 
    Table.Combine(
        {tableOneSource, tableTwoSource, tableThreeSource}
    ),
    renameTableOneColumnOne = 
    Table.RenameColumns(tableOneSource, {"tableOneColumnOne", "ColumnOne"}),
    renameTableTwoColumnOne = 
    Table.RenameColumns(tableTwoSource, {"tableTwoColumnOne", "ColumnOne"}),
    renameTableThreeColumnOne =
    Table.RenameColumns(tableThreeSource, {"tableThreeColumnOne", "ColumnOne"}),
    combineColumnOneRename = 
    Table.Combine({renameTableOneColumnOne, renameTableTwoColumnOne, renameTableThreeColumnOne}),
    groupByColumnOne = 
    Table.Group(
        combineColumnOneRename, 
        {"ColumnOne"}, 
        {
            {"allRows", each _, type table [ColumnOne=nullable text, tableOneColumnTwo=nullable text, tableTwoColumnTwo=nullable text, tableThreeColumnTwo=nullable text]}
        }
    ),
    unpivotNonGroupedColumns = 
    Table.TransformColumns(
        groupByColumnOne, 
        {
            {"allRows", each Table.UnpivotOtherColumns(_, {"ColumnOne"}, "name", "value")}
        }
    ),
    transposeColumns = 
    Table.TransformColumns(
        unpivotNonGroupedColumns, 
        {
            {"allRows", each Table.PromoteHeaders(Table.Transpose(Table.SelectColumns(_, {"name", "value"})))}
        }
    ),
    expandInnerTables = 
    Table.ExpandTableColumn(
        transposeColumns, 
        "allRows", 
        {"tableOneColumnTwo", "tableTwoColumnTwo", "tableThreeColumnTwo"}
    )
in
    expandInnerTables

 Hope this gets you pointed in the right direction.

We can all do well to remember that demoting headers is a great way to solve column name issues amongst multiple tables.

--Nate

Did you try my solution? It does what you are trying to achieve.

--Nate

Sounds like you need to pay a consultant.

--Nate