I did that (flying with service dog) without any issues, by booking two seats, putting my dog's name into the second person's reservation (she even had a passport) and then call CS - but that was back in the good old times when it was still possible to call them.
I`d just book the seats (all three in your name, maybe add "XTRASEAT" as middle name) and then use the chat tool to contact customer service before their IT catches the triple booking.
Check this thread: Purchasing two seats