Re
A tester:
Sub Macro1()
Dim debut As Range
Set debut = Cells(12, 3)
For n = 4 To Cells(12, Columns.Count).End(xlToLeft).Column + 1
If Cells(12, n).Value <> debut.Value Then
Application.DisplayAlerts = False
Range(debut, Cells(12, n - 1)).MergeCells = True
Application.DisplayAlerts = True
Set debut = Cells(12, n)
End If
Next
End Sub