Too little information. Were the 3 seats purchased separately, or on one reservation (using standard EXST practice -- https://pro.delta.com/content/agency...-comfort-.html). The FCM seat buy up offers do not involve any sort of ticket or fare exchange (it will generate separate 006 receipts for FCM purchases, but there is no exchange of original 006 tickets for new ones with new fares). The underlying tickets and fares are not modified and you would still have 3 tickets and you would presumably have to pay buy-up fee on all tickets (you can't purchase FCM seat buy-up offers individually, you must purchase it for all tickets on resevation).