In the third phase of the #100DaysOfGatsby challenge, participants are tasked with integrating WordPress into the existing Gatsby site for the fictional startup AudioC0RE, specifically to power the blog section. Despite the company already using Gatsby and Contentful, the new head of marketing prefers WordPress, prompting a covert plan to incorporate it without the marketing team knowing. The integration is facilitated by Gatsby's WordPress capabilities, allowing for content previews and rapid updates through Incremental Builds. Participants are guided to utilize Gatsby's documentation to implement the new WordPress integration, create a /blog route, and enhance SEO using the Yoast SEO plugin and WPGraphQL. The challenge encourages collaboration and knowledge sharing via Gatsby’s GitHub Discussion forum and a livestream event, reflecting the dynamic and fast-paced environment of a startup.