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.