Поскольку вы установили
<ColumnDefinition Width="*" />
, а ширина каждого столбца будет равна 1/3 ширины экрана при наличии 3 столбцов.
<ScrollView>
<Grid x:Name="MainGrid">
<Grid.RowDefinitions>
<RowDefinition Height="auto" />
<RowDefinition Height="auto" />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="0.33*" />
<ColumnDefinition Width="0.33*" />
<ColumnDefinition Width="0.33*" />
</Grid.ColumnDefinitions>
<Button Text="aaaaaaaaaaaaaaaaaa"
x:Name="btn1"
VerticalOptions="CenterAndExpand"
HorizontalOptions="Center"
HeightRequest="{Binding Width, Source={x:Reference btn1}}"
Grid.Row="0"
Grid.Column="0"/>
<Button Text="Text2..."
x:Name="btn2"
VerticalOptions="CenterAndExpand"
HorizontalOptions="Center"
WidthRequest="{Binding Width, Source={x:Reference btn1}}"
HeightRequest="{Binding Width, Source={x:Reference btn2}}"
Grid.Row="0"
Grid.Column="1"/>
<Button Text="Text3..."
x:Name="btn3"
VerticalOptions="CenterAndExpand"
HorizontalOptions="Center"
WidthRequest="{Binding Width, Source={x:Reference btn1}}"
HeightRequest="{Binding Width, Source={x:Reference btn3}}"
Grid.Row="0"
Grid.Column="2"/>
<Button Text="aaaaaaaaaaaaaaaaa"
x:Name="btn4"
VerticalOptions="CenterAndExpand"
HorizontalOptions="Center"
WidthRequest="{Binding Width, Source={x:Reference btn1}}"
HeightRequest="{Binding Width, Source={x:Reference btn4}}"
Grid.Row="1"
Grid.Column="0"/>
<Button Text="Text5..."
x:Name="btn5"
VerticalOptions="CenterAndExpand"
HorizontalOptions="Center"
WidthRequest="{Binding Width, Source={x:Reference btn1}}"
HeightRequest="{Binding Width, Source={x:Reference btn5}}"
Grid.Row="1"
Grid.Column="1"/>
<Button Text="Text6..."
x:Name="btn6"
VerticalOptions="CenterAndExpand"
HorizontalOptions="Center"
WidthRequest="{Binding Width, Source={x:Reference btn1}}"
HeightRequest="{Binding Width, Source={x:Reference btn6}}"
Grid.Row="1"
Grid.Column="2"/>
</Grid>
</ScrollView>