Assuming the dropdown controls you mentioned are the only ones in the document and your text content control has the title 'Output', you could use code in the document's 'This Document' module like:
Code:
Private Sub Document_ContentControlOnExit(ByVal ContentControl As ContentControl, Cancel As Boolean)
Dim StrOutput As String, CtrlID As String
If ContentControl.Type <> wdContentControlDropdownList Then Exit Sub
For Each ContentControl In ContentControls
If ContentControl.Type = wdContentControlDropdownList Then
If ContentControl.Range.Text <> ContentControl.PlaceholderText Then
StrOutput = StrOutput & ContentControl.Range.Text & "+"
Else
StrOutput = StrOutput & "N/A" & "+"
End If
End If
If ContentControl.Title = "Output" Then CtrlID = ContentControl.ID
Next
StrOutput = Left(StrOutput, Len(StrOutput) - 1)
ContentControls(CtrlID).Range.Text = StrOutput
End Sub