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If I understand correctly, you want to have an additional 3% charge for the payment in this form (http://www.jotform.com/60957484185165)? If yes, then I believe you can use the Tax option to have an additional 3% charge on the payment. Simply follow the steps below.
1. Open the payment wizard on your form builder.
2. Click next until you get to the "Other Options"
3. Select Tax and add a base rate of 3%
4. Save and Finish the setup of the payment wizard.
Here are several guides that can help you get started.
If you need further assistance, please let us know.