The 3 main mermaids, are usualy reffered to as The Girls' . In this case, the girls are Amy, Brenna, and Kelsey. Although many people think that Blaire is considered one of the girls, but she is not. The reason they are considered the girls, and not Blaire, is because they are always together, they keep one big secret, and they were friends with each other, before they met Blaire.
The Girls, were actually changed durring each season, in season 1, Amy is the girl, in the very end of season 1, Brenna and Amy are the girls, and closer to the end of season 2 Kelsey, Brenna and Amy were the girls.In season 3 Blaire is one of the main characters