The best answer to the question: How does the speaker´s metaphor of a gold star in paragraph 3 support the purpose of the speech, would be: It emphasizes the selfish and superficial motivation behind voluntour opportunities.
Explanation:
In this Speech to the Student Body of Evergreen High, on Voluntourism: An Opportunity Too Good to be True, it seems that the author is completely against the idea of voluntourism, equating it to more of an economical, and totally selfish activity, rather than an opportunity to both do something for someone else, and a learning experience. Among the many arguments used against voluntourism, the author states that people who go in these adventures are mostly looking for good points on their resumes, as a way to show experience on a field, and show social interests that will look good. Thus, he insists that much of the motivation behind voluntourism vs volunteering, is actually selfish and superficial, and this idea is supported by people seeking the "gold star" to be placed on their resume.