ShellContent を TabBar の配下に変更

AppShell.xaml

<?xml version="1.0" encoding="UTF-8" ?>
<Shell
    x:Class="MauiApp5.AppShell"
    xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
    xmlns:local="clr-namespace:MauiApp5"
    Shell.FlyoutBehavior="Disabled">

    <TabBar>
        <ShellContent Title="Hello, World!" ContentTemplate="{DataTemplate local:MainPage}" Route="MainPage" />
        <ShellContent Title="Page1" ContentTemplate="{DataTemplate local:Page1}" Route="Page1" />
        <ShellContent Title="Page2" ContentTemplate="{DataTemplate local:Page2}" Route="Page2" />
        <ShellContent Title="Page3" ContentTemplate="{DataTemplate local:Page3}" Route="Page3" />
    </TabBar>
</Shell>