UICollectionView showing 2 items instead of 3

I have implemented my UICollectionView like this

extension MyViewController: UICollectionViewDelegate, UICollectionViewDataSource {
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return photos.count
    }

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "PhotoCell", for: indexPath)
        cell.backgroundColor = .black
        return cell
    }
}

extension MyViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        sizeForItemAt indexPath: IndexPath) -> CGSize {
        let widthPerItem = view.frame.width / 3
        return CGSize(width: widthPerItem, height: widthPerItem)
    }
}

but instead of 3 items in a row I'm getting 2 items

how can i fix it? i didn't set any UIEdgeInset for my items and sections

This is because your screan width can not fit 3 cells. This is because collection view has cell min line spacing property in your storyboard if you try with small width in sizeForItemAt they will fit.

  func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        sizeForItemAt indexPath: IndexPath) -> CGSize {
        let widthPerItem = view.frame.width / 3 - ((collectionViewLayout as? UICollectionViewFlowLayout)?.minimumInterItemSpacing ?? 0.0)
        return CGSize(width: widthPerItem, height: widthPerItem)
    }