When you first use TM3, the invoices are not attached to the emails that are sent out as standard. To add the relevant Invoice to an email you will need to follow the step below.
Navigate to Settings (cog wheel in the top right of TM3) > Documents > Invoice > Click on Invoice Cover Email > View.
This will bring you to the email template editor. From here you can edit the content in the email. To attach the Invoice; click on the paper clip icon > Attach appropriate invoice > Done. This will ensure that when you send this email to someone, it will send the correct invoice.
You will know the invoice is attached as there will be an Invoice attachment on the template editor. Click Save in the top right corner when you are happy with your changes.