Wrong way to build very simple layout (Add your own styles only for special Un-Even Layouts and ideas like this).
First, you can use Webflow CMS Build in Collection list settings (3 cols).
Tutorial
I think the best solution is to follow the steps her (Same layout 2-3 minutes of work):
“THE GAP”:
In your case you set manually the width of each Collection-item - to 300px Width. 3 images in a row (3x300 = 900px) - but the container W is greater > 900