To be honest, this seems more like a quirk than a test, but Sachin Patel spotted Google showing three rows of Google Shopping Ads in the Google Search results. We've seen two rows on mobile search previously and now three rows on desktop search.
Sachin posted this screenshot and video on X - as you can see in the video, he expanded the results, which loaded a new row of Shopping Ads, under the already two rows of shopping ads...
Here is a screenshot:
Forum discussion at X.