FYI, I did a workaround, where I inserted a new column A that serves as a "clear all" checkbox for that row. The remaining three checkbox columns (now B, C, and D) remain mutually exclusive, but not optional.
A user can clear their checkboxes on a row by clicking in the A cell. For example, click A3 clears B33.
Here is the code:
Private Sub Worksheet_SelectionChange(ByVal Target As Range) If Target.Cells.Count > 1 Then Exit Sub
If Target.Column >= 1 And Target.Column <= 5 Then
'Enable mutually-exclusive selection of cells B, C, or D.
'That is, clear B:D and then reset target-selected cell to be
'a Marlett 'a', which displays as a checkmark.
If Not Intersect(Target, Range("B" & Target.Row & ":D" & Target.Row)) Is Nothing Then
Target.Font.Name = "Marlett"
Range("B" & Target.Row & ":D" & Target.Row).ClearContents
If Target = vbNullString Then
Target = "a"
Else
Target = vbNullString
End If
Else
'Enable clicking cell in column A to clear columns B:D
'Column A is Marlett 'r', which displays as an X, but not until it
'is clicked the first time.
If Not Intersect(Target, Range("A" & Target.Row)) Is Nothing Then
Target.Font.Name = "Marlett"
Target = "r"
Range("B" & Target.Row & ":D" & Target.Row).ClearContents
End If
End If
End If
End Sub