Hi,
From what I understand, ACH, or automated clearing house is a system to process bank transfers between banks.
So I suppose that you want to provide the customers with the necessary information at the end of the checkout so that they can do the bank transfer with their bank's app or online tool. Is that correct ?
In that case, you can create a payment method of the type "bank transfer" via the menu System>Payment methods.
In it's "percentage" option, you can write -3 and it will remove 3% off of the total amount if selected.