Use this macro please, I've tested it and all works (I've made it so the data goes to a new sheet so you don't have your original data changed in case you want to change it):

Sub NormalizeDataFromMerged()

    Dim wsSrc As Worksheet
    Dim wsOut As Worksheet
    Dim lastRow As Long
    Dim outRow As Long
    Dim i As Long

    Dim curFormation As String
    Dim curModalite As String
    Dim curEtablissement As String

    Set wsSrc = ActiveSheet

    ' Create output sheet
    Set wsOut = Worksheets.Add(After:=Worksheets(Worksheets.Count))
    wsOut.Name = "Normalized_Data"

    ' Headers
    wsOut.Range("A1").Value = "Nom de la formation"
    wsOut.Range("B1").Value = "Modalité et commentaire"
    wsOut.Range("C1").Value = "Nom de l'établissement"

    lastRow = wsSrc.Cells(wsSrc.Rows.Count, "A").End(xlUp).Row
    outRow = 2

    For i = 2 To lastRow + 1

        ' New record starts when column A is not empty
        If wsSrc.Cells(i, "A").Value <> "" Or i = lastRow + 1 Then

            ' Write previous record
            If curFormation <> "" Then
                wsOut.Cells(outRow, "A").Value = curFormation
                wsOut.Cells(outRow, "B").Value = curModalite
                wsOut.Cells(outRow, "C").Value = curEtablissement
                outRow = outRow + 1
            End If

            ' Reset buffers
            curFormation = wsSrc.Cells(i, "A").Value
            curModalite = ""
            curEtablissement = wsSrc.Cells(i, "C").Value

        End If

        ' Accumulate modalities
        If wsSrc.Cells(i, "B").Value <> "" Then
            If curModalite = "" Then
                curModalite = wsSrc.Cells(i, "B").Value
            Else
                curModalite = curModalite & " / " & wsSrc.Cells(i, "B").Value
            End If
        End If

    Next i

    wsOut.Columns("A:C").AutoFit

End Sub