This little macro will do it:
Sub Stack()
Application.ScreenUpdating = False
Sheets("Sheet4").Select
Range("A1:B1", Selection.End(xlDown)).ClearContents
'Copy Sheet1
Sheets("Sheet1").Select
Range("A1:B1", Selection.End(xlDown)).Copy
Sheets("Sheet4").Select
Range("A1").Select
ActiveSheet.Paste
Application.CutCopyMode = False
'Copy Sheet2
Sheets("Sheet2").Select
Range("A1:B1", Selection.End(xlDown)).Copy
Sheets("Sheet4").Select
Columns("A:A").SpecialCells(xlCellTypeBlanks)(1, 1).Select
ActiveSheet.Paste
Application.CutCopyMode = False
'Copy Sheet3
Sheets("Sheet3").Select
Range("A1:B1", Selection.End(xlDown)).Copy
Sheets("Sheet4").Select
Columns("A:A").SpecialCells(xlCellTypeBlanks)(1, 1).Select
ActiveSheet.Paste
Application.CutCopyMode = False
Range("A1").Select
End Sub
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