The below screen is how the digital version of an invoice appears in TM3, we are looking to apply a discount to this invoice so the section that we are going to need to access is the 'More Actions' button.
When selecting the 'More Actions' button, a pop up window will appear and you need to select 'Apply Discount'.
This will bring up the below window where you can choose to add a 'Percentage' or 'Total Amount' of discount. I have chosen to add a 15% discount; TM3 will calculate this for you.
Once you are happy with the value, click 'Accept' and the discount will be added to the invoice.