Combo ListIndex?

Private Sub UserForm_Initialize()
  Dim r As Range
  ComboBox1.ColumnCount = 2
  ActiveWorkbook.Names.Add Name:="MyRange", RefersToR1C1:= _
    "=OFFSET(Sheet1!R1C1,1,0,COUNTA(Sheet1!C1),2)"
      
  ComboBox1.RowSource = Range("MyRange").Address
  
  Set r = Range("MyRange").Find("AG5", , xlValues)
  ComboBox1.RowSource = Range(r.Offset(0, -1).Address, Cells(Rows.Count, 1).End(xlUp).Address).Address
  ComboBox1.ListIndex = 0
End Sub

Option Explicit
'Public cCB() As Control, cTB() As Control 'In a Module
Private Sub UserForm_Initialize()
  FillControlArrays
End Sub

Private Sub ComboBox1_Change()
  FillTBs
End Sub

Private Sub ComboBox2_Change()
  FillTBs
End Sub

Private Sub ComboBox3_Change()
  FillTBs
End Sub

Private Sub FillControlArrays()
  Dim iCB As Integer, iTB As Integer, c As Control
  For Each c In Me.Controls
    Select Case TypeName(c)
      Case "TextBox"
        iTB = iTB + 1
        ReDim Preserve cTB(1 To iTB)
        Set cTB(iTB) = c
       Case "ComboBox"
        iCB = iCB + 1
        ReDim Preserve cCB(1 To iCB)
        Set cCB(iCB) = c
      End Select
    Next c
End Sub

Private Sub FillTBs()
Dim offset As Integer, i As Integer, j As Integer
  
  'Set offsets for textbox controls based on combobox
  Select Case ActiveControl
    Case ComboBox1
      offset = 0
    Case ComboBox2
      offset = 5
    Case ComboBox3
      offset = 10
    Case Else
      offset = -1
  End Select
      
  If offset = -1 Then Exit Sub
  'Fill TextBox controls will values based on combobox that is active and offset
  For i = 1 To 5
    j = i + offset
    cTB(j) = ActiveControl.Value & i
  Next i
End Sub