I said 7 but the arguments for 3 make some sense. She does live behind a facade and lacks a strong sense of identity but 3s are more purposeful in their pursuit of success. I think Holly is less interested in success and more in the freedom to enjoy the pleasures of life. Its not high achievement she pursues but the benefits that come from it - success is just a means to an end for her. I think she has the spontaneous, fun loving, scattered, optimistic nature of a 7 - she's not serious and focused enough to be a 3.
I'm still inclined to say ENFP 7w6 so/sx. Here's a good 7w6 description that really seems to fit Holly:
Source7w6s are less grounded and more visibly anxious. They are more manic. They are much more prone to bipolar personality disorder. They are mercurial and endearing. They tend to be more amiable and friendly. They are natural peter pan types who never grow up. They are the more childlike side of seven. They are more creative and fantastical. They are more excited about life and never stop dreaming. They have an attitude of wide-eyed wonderment towards the world. They are prone to blurring the distinction between fantasy and reality. They are much more "pie in the sky" thinkers. They believe in "potential" and that is a big theme in their lives. They believe in potential in others and pockets of potential in the world. They believe the "holy grail" is out there somewhere and it's just a matter of finding it. They are more likely to chase their own tail. They lack the pragmatism and realism of their seven-wing-eight siblings.
7w6s feel more of a need to "check in" with others. Their six wing gives them a need to establish "solidarity" with people they care about. They care about how they are seen. The dense party animal stereotype offends them more. They don't like to be taken for granted as someone who is happy all the time and doesn't have problems. They want to be seen as someone who has depth of personality. They want to be seen as someone who is also "human" and can relate to how you are feeling. Their six wing causes them to be awed and apalled. They have more highs and lows. They have a good-natured sense of humor and are more natural entertainers. They tend to be funny more than they are characters.
7w6s tend to fear getting "trapped" in a rut as opposed to stuck in a rut. Their six wing makes them more likely to feel defeatism and despair. They admit to themselves the rut has "got" them in some way. They can see the walls closing in and becoming more trapped if they don't do something.