Sang-woo's mom is the old lady with the gray hair. The middle-aged woman that they met at the airport is the mom of Sae-byeok and the boy.
They did have a Sae-byeok cameo for like three seconds in episode 5 of season 3 lol. When Gi-hun is going to stab the other players, he sees a vision of her telling him not to do it because he's "not that kind of person".
That vision is a callback to season 1 when she told him the same thing when he was going to stab Sang-woo before the final round.
