FYI...Multiple ETCs can be used for a single booking, it is one up to the number of passengers on the itinerary. So if you were only rebooking one person, then yes, the OP would be correct, but if rebooking all 3 pax on one reservation, it's not a problem.

On the payment page, you select the voucher as payment method, then have the option to assign it to any if the passengers on the itinerary. The total due will reduce by the amount of the first voucher. Once applied, you can repeat this, with the option to assign the voucher to any additional passengers in the itin (someone other than the person you selected for the first). And so on.