The third sex scene is an either/or sequence, based on whether you're running Raven's romance path or her normal path. Raven's a party-girl, and casual sex has long been a part of her life. To activate her romance path you need to not screw her immediately. It's just a case of different text, but same graphics.
Here's a full set of spank bank entries if you just want to check it out; just copy them into the Savegame folder:
Windows: %USERPROFILE%/AppData/LocalLow/FlexibleMedia/Paradise Lust
OSX: ~/Library/Application Support/FlexibleMedia/Paradise Lust
Linux: $XDG_CONFIG_HOME/unity3d/FlexibleMedia/Paradise Lust