Maybe change the .Text part to .Value when the value of combobox3 changes.

Sorry, the code tags don't seem to work.

Private Sub ComboBox3_Change()
Dim objMsg As Outlook.MailItem
Dim OSubject As String
Set objMsg = Outlook.ActiveInspector.CurrentItem
objMsg.Subject = Left(objMsg.Subject, 16) & ComboBox3.Value
End Sub