1) Remove the Tags from XFLOW A+B+C and put next code on top of CommandButton1_Click.
Code:
    For Each ctl In Frame1.Controls
        If TypeName(ctl) = "OptionButton" Then
            If ctl.Value = False Then x = x + 1
        End If
    Next
    If x = 3 Then MsgBox "Select a machine before you write data": Exit Sub
2) In designmode set the Enabled-property of the CB's and both TB's to False.