Scar is not their father, their father is a rouge. Kovu was chosen by Scar but Kovu is not his son or the other two.

^Yes, that. And as Kovu was the one Scar chose as his heir, he would be the only one Scar cared about. I don't think Scar loved him at all, though.

nah the only thing he really cared about was being king

Even if Kovu wasn't Scar's real cub , Scar choosed him to become the next king and so loved him the most I guess , but also loved Vitani ... and Nuka a bit.