For that, you could use code like:
Sub MakeCheckBoxesExclusive()
Dim oField As FormField, i As Long, j As Long
For Each oField In Selection.Frames(1).Range.FormFields
With oField
If .Type = wdFieldFormCheckBox Then
i = i + 1
If .Range.Start = Selection.FormFields(1).Range.Start Then
j = i
.CheckBox.Value = True
Else
.CheckBox.Value = False
End If
End If
End With
Next oField
Select Case j
Case 1
'Do whatever is required if the 1st checkbox is checked
MsgBox "Do whatever is required if the 1st checkbox is checked"
Case 2
MsgBox "Do whatever is required if the 2nd checkbox is checked"
Case 3
MsgBox "Do whatever is required if the 3rd checkbox is checked"
Case Else
MsgBox "Do whatever is required if any other checkbox is checked"
End Select
End Sub