While what you ask is possible. It requires a lot of error checking to ensure that the count is a true reflection of the selections. It requires the user to tab through each to increment the values. Simply leaving the values at their defaults will not increment the count.
I believe it would make more sense to use a macro to count the values after they have all been completed. That way there should be no false values. e.g.
Code:
Sub CountValues()
Dim A_Count As Long
Dim B_Count As Long
Dim C_Count As Long
Dim oFF As FormField
A_Count = 0
B_Count = 0
C_Count = 0
For Each oFF In ActiveDocument.FormFields
If oFF.Type = wdFieldFormDropDown Then
Select Case oFF.Result
Case "A": A_Count = A_Count + 1
Case "B": B_Count = B_Count + 1
Case "C": C_Count = C_Count + 1
Case Else
End Select
End If
Next oFF
MsgBox "A - Selected " & A_Count & vbCr & _
"B - Selected " & B_Count & vbCr & _
"C - Selected " & C_Count
lbl_Exit:
Exit Sub
End Sub