Can someone please suggest and explain the best settings for gain/expo/sensitivity etc for the phantom 3 to achieve smooth, cinematic looking footage. Thanks

I have only reduced the YAW expo to its lowest value to help smooth it out.  Mostly it takes practice.  First you must be very comfortable to fly FPV off the screen, a full sized iPad helps.  Then a big space with a moving subject is good to practice on.  Here is an example video of one of my "practice" flights.

Leave the GAINS as they are. Don't mess with this unless your bird is flying wobbly or you're getting bad vibrations.
To get smooth YAW, Go in Gain & Expo Tuning > change your RUDDER to the lowest setting of 0.20.

Agree with quanthonytrang; lower Expo for Rudder to 0.2. Next, lengthen your sticks (they are adjustable); this will give you more control.
Have fun!