The other three girlfriends are met online from the dating websites, as
They are merely a fun alternative to the game that offer rewards for dating them. You dont have to date any, but some have good rewards. Each girl becomes available as you progress through the game but not all of them are aquired through missions.
Each woman has different tastes and will respond accordingly.
Reaching 100% fondness with all five girlfriends is not required to achieve 100% completion of the game.
Each girl has a blue arrow above their heads and will be standing around doing various things, like talking to someone or shooting a gun.
Sex appeal is very important when getting a girlfriend and turning up to a date.