I love third sittings with models!  

• First Sitting:  the objective of any first sitting with a new-to-me model is "get acquainted".  Although I typically have seen pictures of that model before the sitting, I really don't know how the model moves, how she will appear under my style of lighting, how she takes direction, or what our chemistry will be like.  Usually, for the first sitting, I plan basic setups and just concentrate on getting to know the model.
• Second Sitting:  When I engage a model, I typically have several ideas of things to try out, and usually we don't get to all those ideas during the first sitting.  During the second sitting, we typically finish that initial list of ideas, and we work on getting comfortable with each other.
• Third Sitting:  By now, we are comfortable with each other, and we have seen the results from the previous sittings.  Frankly, I feel that in order to entice the model to multiple sittings, I have to produce good results from the initial sittings.  By the third sitting, I am more willing to experiment & try new things, and often so is the model.  

So, the third, fourth, fifth sitting are usually the most fun.  We both feel less inhibited, we try new things, and we relax and work quickly.