NavigationControllerで何回かpushしたあとのViewControllerで2つ以上ViewControllerをpopして戻る

2回戻る例。

ViewController.swift

guard let navigationController = navigationController else {
    return
}
let viewControllers: [UIViewController] = navigationController.viewControllers as [UIViewController]
navigationController.popToViewController(viewControllers[viewControllers.count - 3], animated: true)

...1回だけ戻る例も載せておきます。

ViewController.swift

guard let switchViewController = navigationController?.viewControllers[1] else {
    return
}
navigationController?.popToViewController(switchViewController, animated: true)

参考

Pop 2 view controllers in Nav Controller in Swift
http://stackoverflow.com/questions/26132658/pop-2-view-controllers-in-nav-controller-in-swift

Swift popToViewController
http://stackoverflow.com/questions/28190070/swift-poptoviewcontroller