The other three, Carmen Ortiz, Kiki Jenkins, and Alexandra Chilton, are met via online dating services.
In The Ballad of Gay Tony, Luis meets the temporary girlfriends within the dancefloor of Maisonette 9.
Call your friend and perform activities with him on a regular basis to increase your 'like stat'.
In order to unlock a friend's special ability, you must have a high 'like stat' for that friend, and this stat is based on how regularly you hang out with your friend.
Catalina is inside The Welcome Pump in Dillimore and is threatening two men with a knife.
Carl enters the room and explains to her that he is looking for a friend's cousin, who he discovers is Catalina.
Dating any of the six girlfriends is not required for 100% completion of the game.
Denise is first encountered during the mission Burning Desire for Frank Tenpenny and Millie first appears during the mission Key To Her Heart in Las Venturas.