There was a change recently to the way Google displayed the Products section in Google My Business.
At first glance, this was just a change to the layout but it also changed how Products are populated in the Knowledge Panel. Previously, you could control which products showed up in the Knowledge Panel (it only shows 3) by re-arranging them to move the ones you wanted to the top.
This is no longer an option. Google confirmed that the ones that appear are the ones that were edited most recently. Thus, you can influence the order by editing the ones that you want to show first.