Google is working to reduce the number of Product Listing Ads (PLAs) which shows at the top of the search results page. Usually one can see up to 6 items and often a carousel, so it would be a bit unusual to see only three items, when more are available.
There is one thing which I find interesting that when you see only three ads instead of six, the added white space makes those really stand out.
You can see the difference if you compare them with each other.
Google tested it back in May, but at that time it seemed to be a limited test. It may be bringing them back.