Try this:

Private Sub Worksheet_Change(ByVal Target As Range)

Dim NewValue As String
If Target.Column = 1 Then
Select Case Target.Text
Case Is = "005", "5"
NewValue = "AAA"
Case Is = "102"
NewValue = "BBB"
Case Is = "109"
NewValue = "CCC"
Case Is = "112"
NewValue = "DDD"
Case Is = "641"
NewValue = "EEE"
Case Is = "2090"
NewValue = "FFF"
End Select
Target.Offset(0, 1) = NewValue
End If

End Sub

This assumes that you want the Triple letters to occurr right next to
your exixting numbers.