We have requirement to apply discount at header level but that should come as a line item at line le

We have requirement to apply discount at header level but that should come as a line item at line level and show as discount to the customer e.g. I have two line items:

Item A - $100

Item B - $200

Header Total: $300

Now total price is $300 but when i apply this header discount of 3% then below should happen:

Item A - $100

Item B - $200

Dummy Item - (-$9)

Header Total: $291

Pls let me know easier way to achieve this

Regards,

MK