I have 3 radio button options to chose from, but I only want one certain one to calculate a sales tax. How can I assign a calculation to one radio button?

OK. Let's say the name of the radio-button group is "Radio1", the export value of that field is "3", the name of the sub-total is "SubTotal" and the tax rate is 16%. You can use this code as the custom calculation script of the Tax field:

event.value = (this.getField("Radio1").valueAsString=="3") ? Number(this.getField("SubTotal").valueAsString) * 0.16 : 0;