Yes, that's a relevant additional piece of information. You can amend or cancel a booking within 24 hours without fee. However if the price changes as a result of 3 people being on the booking you will have to pay the extra not just for the additional person but also for the initial 2 passengers. You would need to call the contact centre to make this change, other than that a cancellation should be possible online - only proceed if it shows a full refund and details on the second screen.