Atlases seem to work the same way in the regular layout in 3.0 as they did in the print composer of 2.18. But the report layout works a little differently, and I'm still trying to figure it out.
I added two field group sections to my report, and set up each section as the atlas was set up in my 2.18 project. When I exported the report, it had all the pages I expected, just as if I had created atlases in two different layouts.
Now I want to preview each page of the report and make sure it looks okay before exporting the report again. In 2.18 and in the layout of 3.0, there's an Atlas toolbar that let you preview the atlas and flip through the pages. I don't see any way to use the Atlas toolbar in a report, though.
How do I preview the pages of a field group section in a report in QGIS 3.0?