We’ve seen reports and are also seeing some AdWords ads displaying with three lines of sitelinks, and Google has updated its help documents to indicate that “up to six” and “three-line” Ad Sitelinks are now being displayed.
Previously, Ad Sitelinks were only displayed in one-line, two-line and embedded formats. Generally speaking, the multi-line AdSitelinks are only displayed when the ad is deemed an ideal match for the user’s search query, and is usually triggered by a query that includes a brand name.
Sitelinks are an AdWords extension that permits advertisers to include up to 10 direct links to internal pages on their sites, and Google then chooses which of those to display for a particular query. Google also recently expanded the number of Sitelinks that display in organic search results, as well.