Mixed Layout: 2 Small Posts, Followed by 1 Full
In order to show your blog posts in a grid format, go to Appearance > Customize > Blog Feed and change the Post Display Format.
This is a mixed format display and shows two posts in one row (side by side) followed by a full format post. So if you have set your settings to show 6 posts (“Blog pages show at most”) in the blog feed under Settings > Reading, then they would be shown as: 2 side by side, 1 full, 2 side by side, 1 full.
More on the different post layouts can be found here.