A sample code to show custom width/ height proportion in stackview

stackview.playground

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    override func loadView() {
        let view = UIView()
        view.backgroundColor = .white
        self.view = view
        setUpView()
        //setUpViewVertical()
    }
    
    private func setUpView() {
        let viewStack = createAStackViewWith(axis: .horizontal)
        let view1 = createAUIView(with: .red)
        let view2 = createAUIView(with: .green)
        let view3 = createAUIView(with: .blue)
        view.addSubview(viewStack)
        viewStack.addArrangedSubview(view1)
        viewStack.addArrangedSubview(view2)
        viewStack.addArrangedSubview(view3)
        NSLayoutConstraint.activate([
        
            viewStack.leftAnchor.constraint(equalTo: view.leftAnchor),
            viewStack.rightAnchor.constraint(equalTo: view.rightAnchor),
            viewStack.topAnchor.constraint(equalTo: view.topAnchor, constant: 44),
            viewStack.heightAnchor.constraint(equalToConstant: 44),
            
            view1.widthAnchor.constraint(equalTo: viewStack.widthAnchor, multiplier: 0.50),
            view2.widthAnchor.constraint(equalTo: viewStack.widthAnchor, multiplier: 0.30),
            view3.widthAnchor.constraint(equalTo: viewStack.widthAnchor, multiplier: 0.20),
        
        ])
    }
    private func setUpViewVertical() {
        let viewStack = createAStackViewWith(axis: .vertical)
        let view1 = createAUIView(with: .red)
        let view2 = createAUIView(with: .green)
        let view3 = createAUIView(with: .blue)
        view.addSubview(viewStack)
        viewStack.addArrangedSubview(view1)
        viewStack.addArrangedSubview(view2)
        viewStack.addArrangedSubview(view3)
        NSLayoutConstraint.activate([
        
            viewStack.leftAnchor.constraint(equalTo: view.leftAnchor),
            viewStack.rightAnchor.constraint(equalTo: view.rightAnchor),
            viewStack.topAnchor.constraint(equalTo: view.topAnchor),
            viewStack.bottomAnchor.constraint(equalTo: view.bottomAnchor),
            
            view1.heightAnchor.constraint(equalTo: viewStack.heightAnchor, multiplier: 0.50),
            view2.heightAnchor.constraint(equalTo: viewStack.heightAnchor, multiplier: 0.30),
            view3.heightAnchor.constraint(equalTo: viewStack.heightAnchor, multiplier: 0.20),
        
        ])
    }
    private func createAStackViewWith(axis: NSLayoutConstraint.Axis) -> UIStackView {
        let stackView = UIStackView()
        stackView.axis = axis
        stackView.alignment = .fill
        stackView.translatesAutoresizingMaskIntoConstraints = false
        stackView.distribution = .fillProportionally
        return stackView
    }
    
    private func createAUIView(with backgroundColor: UIColor) -> UIView {
        let newView = UIView()
        newView.backgroundColor = backgroundColor
//        newView.layer.borderColor = UIColor.white.cgColor
//        newView.layer.borderWidth = 5
        newView.translatesAutoresizingMaskIntoConstraints = false
        return newView
    }
}
// Present the view controller in the Live View window
PlaygroundPage.current.liveView = MyViewController()