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If there are only three of you on the booking, then it sounds like it should be a normal booking so you should be able to get split off into a separate booking/PNR and then UuA. You will need to phone. But when you do, before you actually go ahead with the split do make sure that your fare is one that allows you to UuA. Although you can UuA all published fares (providing you're not booked into Q, O or G booking classes), IIRC some contract/negotiated/net fares don't allow UuA.