What are three topics addressed by The Night Circus, and what theme do they portray?

I believe that the question wants three motifs/symbols/etc. that illustrate a single theme.  That's definitely doable with The Night Circus.  The book is a visual banquet, and it's loaded with themes.  
Three topics that the book addresses are power, magical reality, and eternity.  All three of those topics are used for various events during the book, but all three of them do help illustrate the book's theme of true love.   
Celia Bowen and Marco Alisdair are both apprentice magicians who are supposed to battle each other to the death.  Fortunately, they fall in love with each other and find a way to both survive despite the rules of the magical bet placed between Alexander and Hector Bowen. 
Both Celia and Marco have the power to destroy each other.  However, instead of using their magic for death, they use their magical powers to bring surreal fantasy images into reality.  They do this as a form of courtship.  Celia and Marco both use the confines of the circus in order to dazzle and delight each other.  As long as both characters are alive and take care of the circus, the night circus will continue.  In other words, as long as the two of them remain in love, they will not age.  Their relationship is eternal. 
At the end of the book, Celia and Marco find a way to continue being in love forever outside of the circus.  Marco, Celia, and their actions illustrate the theme of love and some common beliefs about love.  Beliefs like the following which come from the culture studies course that I teach.
True love is the height of existence
True love is eternal
True love is a powerful and unstoppable force
There is only a single "one and only"
Love transcends all boundaries
Love survives all problems
Love is mysterious