Aligning collection view cells to fit exactly 3 per row

1- You have to conform to UICollectionViewDelegateFlowLayout

2- You should implement collectionView(_:layout:sizeForItemAt:) method, as follows:

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let screenWidth = UIScreen.main.bounds.width
    let scaleFactor = (screenWidth / 3) - 6

    return CGSize(width: scaleFactor, height: scaleFactor)
}

Assuming that the minimum spaces are 8 -for example-,

the output should be three items for each row, square sized.

Remark: If you want to set the spaces between the cells to be zero, then scaleFactor should be:

let scaleFactor = (screenWidth / 3)

Hope this helped.

you can use this

This code is somehow written that you can change section inset or minimumInteritemSpacing and this calculate and resize with this parameters

you can use this code or download project from Github

 func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {


        let flowLayout = collectionViewLayout as! UICollectionViewFlowLayout

        let numberofItem: CGFloat = 3

        let collectionViewWidth = self.collectionView.bounds.width

        let extraSpace = (numberofItem - 1) * flowLayout.minimumInteritemSpacing

        let inset = flowLayout.sectionInset.right + flowLayout.sectionInset.left

        let width = Int((collectionViewWidth - extraSpace - inset) / numberofItem)

        print(width)

        return CGSize(width: width, height: width)
    }

You have to implement the

UICollectionViewDelegateFlowLayout

for the spacing stuff.

Set the size of the collectionViewCells like this:

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let yourWidth = collectionView.bounds.width/3.0
    let yourHeight = yourWidth

    return CGSize(width: yourWidth, height: yourHeight)
}

You can also add these functions to get your spacing of the collectionViewCells correct:

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsets.zero
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
    return 0
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 0
}

Swift - Version 4.2

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {

    let noOfCellsInRow = 3

    let flowLayout = collectionViewLayout as! UICollectionViewFlowLayout

    let totalSpace = flowLayout.sectionInset.left
        + flowLayout.sectionInset.right
        + (flowLayout.minimumInteritemSpacing * CGFloat(noOfCellsInRow - 1))

    let size = Int((collectionView.bounds.width - totalSpace) / CGFloat(noOfCellsInRow))

    return CGSize(width: size, height: size) 
}