You are unable to delete an invoice in TM3 if it has a charge on it so firstly you should delete/remove the charge, like the image below.
Once this has been removed the invoice will look like the one in the image below. You can see there is no charge or payment on the invoice and so that is why the Delete Invoice button is available.
When you click delete, a warning message appears like the one in the following image. Click yes to delete or no to cancel.