What is the signifiacance of the title of the Book the Third? In what earlier scene did Dickens refer to an approaching storm?Need Question...

The title of the third book, "The Track of the Storm", is a metaphor where the "storm" which is referred to is the French Revolution that has "broken out" like a storm in Chapter 21 of Book 2, entitled "Echoing Footsteps". Thus the last book attempts to "track" this storm, noting how it develops and its course, and in particular how it threatens to engulf the main characters of the novel, in particular Charles Darnay and his family.

You are right in identifying that this storm imagery comes elsewhere. In fact, it comes in Chapter 21, just before the "outbreak" of this storm with the storming of the Bastille. Lucie is described in her married state as listening to the echoes of footsteps, both of her family and of people in the steet outside their house, but as this section of the Chapter comes to an end, Dickens relates:

But, there were other echoes, from a distance, that rumbled menacingly in the corner all through this space of time. And it was now, about little Lucie´s sixth birthday, that they began to have an awful sound, as of a great storm in France with a dreadful sea rising.

Note how this section introduces two important images that are continued throughout the rest of the book to describe the Revolution - firstly the storm, and secondly the sea, which is used in the latter half of Chapter 21 to describe the uprising of the "patriots" as they storm the Bastille and wreak their terrible revenge.