I\'ve got a UICollectionView, which works ok, until I start scrolling.
Here some pics first:
I think I understand the problem. I'll try and make you understand it too.
If you look closely, then you will see that this issue happens only gradually and not just on the first page swipe.

If I understand correctly, in your app, currently, every UICollectionView item are those rounded boxes which we see, and you have some offset/margin between all of them which is constant. This is what is causing the issue.
Instead, what you should do, is make a UICollectionView item which is 1/3rd of the width of the whole view, and then add that rounded image view inside it. To refer to the image, the green colour should be your UICollectionViewItem and not the black one.