This will work perfect on Android and iOS. However, on WinPhone (at least 8.1Silverlight) it will not.
On WinPhone, the first time pressing the pop-button on page3 have no effect. The second time BOTH page3 and page2 will be popped, landing on MainPage.
This was not the case in earlier versions of Xamarin.Forms. It was introduced in the latest version, 4.0.3 (or if it was the version before).
A way to get around it is wrapping page2 inside a NavigationPage (PushModalAsync(new NavigationPage(new Page2())), but this shouldn't be needed, should it?
Can anyone confirm if this is a bug or not? Is it known? Will it be fixed?