Option Explicit
Option Base 1
 
Private Sub Text1_Change(Index As Integer)
  Dim I As Integer
 
  If Sgn(Len(Trim$(Text1(1).Text))) + Sgn(Len(Trim$(Text1(2).Text))) + _
    Sgn(Len(Trim$(Text1(3).Text))) + Sgn(Len(Trim$(Text1(4).Text))) < 3 Then
    For I = 1 To 4
      ' TextBox wieder freischalten
      Text1(I).Enabled = True
      Text1(I).BackColor = &H80000005
    Next I
  End If
End Sub
 
Private Sub Text1_LostFocus(Index As Integer)
  ' Jenachdem welche 3 Textboxen gefüllt sind, die 4. sperren
  If Text1(2).Text <> "" And Text1(3).Text <> "" And Text1(4).Text <> "" Then
    Text1(1).BackColor = &H8000000F
    Text1(1).Enabled = False
  ElseIf Text1(1).Text <> "" And Text1(3).Text <> "" And Text1(4).Text <> "" Then
    Text1(2).BackColor = &H8000000F
    Text1(2).Enabled = False
    ElseIf Text1(1).Text <> "" And Text1(2).Text <> "" And Text1(4).Text <> "" Then
    Text1(3).BackColor = &H8000000F
    Text1(3).Enabled = False
    ElseIf Text1(1).Text <> "" And Text1(2).Text <> "" And Text1(3).Text <> "" Then
    Text1(4).BackColor = &H8000000F
    Text1(4).Enabled = False
    End If
End Sub