What is the significance of the title "Book the Third," and in what earlier scene does Dickens refer to an approaching storm?I need this question...

With the religious theme of redemption and with Carton as a Christ-like figure, the number 3 plays a recurring role in Dickens's "A Tale of Two Cities."  In Book the First, Sydney Carton is introduced to his dual, Charles Darnay, and by contrast to him, Sydney becomes more aware of his sin of dissipation (of drinking and of living to his potential). Then, in Book the Second, Carton awakens feelings in himself that he has believed suppressed; he pledges his love to Lucie and his friendship to Darnay.  Now, in Book the Third, in the storm of French Revolution and its blood lust, Carton fulfills his promises to the Darnay family and becomes the sacrificial lamb, who in his sacrifice, redeems himself.

The events of the third book are foreshadowed by the chapter "Hundreds of People" when Carton remarks to Lucie,

There is a great crowd coming one day into our lives, if that be so...The footsteps destined to come to all of us, Miss Manette, or are we to divide them among us?

Later, in Chapter 21 "Echoing Footsteps," this approaching storm is again foreshadowed in Book the Second:

Among the echoes, then, there would arise the sound of footsteps at her own [Lucie's] early grave; and thoughts of the husband who would be left so desolate, and who would mourn so much for her so much, swelled to her eyes  and broke like waves.

Chapters 22 and 23 further this motif as in "The Sea Still Rises" the Vengeance emerges with men who "were terrible, in the bloody-minded anger with which they looked from windows."  The crowd captures Foulon, an aristocrat who has suggested that the peasants eat grass, and they hang the man as "the blood and hurry had not changed."

In the next chapter, "Fire Rises," "lay a ruined country, yielding nothing but desolation..."  The chateau of the Marquis d'Evremonde is set fire; other fires follow and

The altitude of the gallows that would turn to water and quench it, no functionary, by any stretch of mathematics, was able to calculate successfully.

Thus, the storm of vengeance against the aristocrats in the French Revolution is sensed by Carton and Lucie, and it is later begun with single incidences that culminate in the fomented masses who guillotine aristocrats in the French Revolution, the "storm" of Book the Third.