I had the same problem. In the App.Shell.xaml I had
<ShellContent
Title="Home Page"
ContentTemplate="{DataTemplate local:HomePage}"
Route="HomePage"
/>
And in the App.Shell.xaml.cs I had
Routing.RegisterRoute(nameof(HomePage), typeof(HomePage));
When I commented this out
//Routing.RegisterRoute(nameof(HomePage), typeof(HomePage));
The page back buttons worked on the third page.
Is your Page1 the start page like my HomePage?