Sub Auffuellen()
Workbooks(1).Worksheets(1).Range("A1:A5").Replace what:="", replacement:="00:00:00", searchorder:=xlByColumns, MatchCase:=True
End Sub

Option Explicit
Sub Auffuellen()
Call EventsOff
Dim puffer1 As Variant, puffer2 As Variant, puffer3 As Variant
Dim zaehler As Long, zeile As Long
zeile = ThisWorkbook.ActiveSheet.Cells(Rows.Count, 4).End(xlUp).Row
ReDim matrix(zeile, 3) As Variant
matrix() = Range(Cells(1, 1), Cells(zeile, 3))
For zaehler = 4 To zeile
If matrix(zaehler, 1) <> "" Then
puffer1 = matrix(zaehler, 1)
Else
matrix(zaehler, 1) = puffer1
End If
If matrix(zaehler, 2) <> "" Then
puffer2 = matrix(zaehler, 2)
Else
matrix(zaehler, 2) = puffer2
End If
If matrix(zaehler, 3) <> "" Then
puffer3 = matrix(zaehler, 3)
Else
matrix(zaehler, 3) = puffer3
End If
Next zaehler
ThisWorkbook.ActiveSheet.Range(Cells(1, 1), Cells(zeile, 3)) = matrix()
Call EventsOn
End Sub

Public Sub EventsOff()
With Application
.ScreenUpdating = False
.EnableEvents = False
.Calculation = xlCalculationManual
End With
End Sub

Public Sub EventsOn()
With Application
.ScreenUpdating = True
.EnableEvents = True
.Calculation = xlCalculationAutomatic
End With
End Sub