try:
Sub CopyEmptyCellsOver3()
Dim rRange As Range
Dim z As Range
On Error Resume Next
Set rRange = Application.InputBox(Prompt:="Select Destination Column (top left cell will be used)", Default:=ActiveCell.Address, Type:=8)
On Error GoTo 0
Set rRange = Intersect(ActiveSheet.UsedRange, rRange.Cells(1).EntireColumn)
Set z = Application.InputBox("Select Source column (top left cell will be used)", "Select", , , , , , 8)
rRange.SpecialCells(xlCellTypeBlanks).FormulaR1C1 = "=RC" & z.Column
rRange.Value = rRange.Value
End Sub