One theme revealed in Volume III of Pride of Prejudice is the need for both personal change and humility. Visiting Pemberley with her aunt and uncle Gardiner was a very humbling experience for Elizabeth. While Elizabeth had begun to understand how wrong she was in her judgements of Darcy while reading his letter, it is while visiting Pemberley that her realization hits her in full force. Darcy's housekeeper's praise of her master as a gentle, kind, caring person makes her understand fully just how wrong she had been.
Humility is also exhibited by Darcy when he welcomes and converses with Elizabeth's aunt and uncle Gardiner from Cheapside, London. When he asked Elizabeth to introduce her relations to him, Elizabeth noticed that "he was surprised by the connexion," but "sustained" his composure "with fortitude" and conversed with them, even inviting Mr. Gardiner to fish on his estate (Ch. 1, Vol. 3). Darcy's show of friendship to Elizabeth's working class relations shows us just how much humility Darcy has gained since Elizabeth chastised him for his pride.
Another theme found in Volume III is want of propriety. Lydia demonstrates a want of propriety in running off with Wickham, which jeopardized her reputation but also her entire family's reputation. Not only that, Mr. Bennet demonstrates a want in propriety by never restraining his youngest daughters, as we see in the passage: "Her father, contented with laughing at them, would never exert himself to restrain the wild giddiness of his youngest daughters" (Ch. 14, Vol. 2).
A third theme deals with the dangers of being easily influenced. We especially see this demonstrated through Bingley who is so easily influenced by Darcy. In fact, Darcy refers to Bingley as being so "unaffectedly modest," meaning so genuinely modest, that he has no confidence in "his own judgement" and instead relies heavily on Darcy's own opinions (Ch. 16, Vol. 3). Bingley's "modesty" is the reason that he allowed Darcy to persuade him into dropping his interest in Jane. Hence we see through Volume III that another theme of Austen's points out the folly of being easily persuaded.