How to put those 3 buttons on the center of bottom? - xaml

Question

In UWP xaml design as below, I split the whole screen to 3 columns and 3 rows. And For row 2 column 1, I need to put a canvas which contains a RelativePanel inside of it. And there will some buttons inside of the panel. My purpose is to make an animation on the panel.
I hope to put those 3 buttons on the bottom/center of the screen, but failed. My layout picture is attached, but it is not what I want.
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="10*"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="2*" />
<RowDefinition Height="15*" />
<RowDefinition Height="3*"/>
</Grid.RowDefinitions>
<Canvas x:Name="toolbarCanvas" Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="3" HorizontalAlignment="Center">
<RelativePanel x:Name="toolbarPanel">
<Button x:Name="bt1" Margin="50,0,50,0" Height="100" Width="100" VerticalAlignment="Center" RelativePanel.LeftOf="bt2"/>
<Button x:Name="bt2" Margin="50,0,50,0" Height="100" Width="100" VerticalAlignment="Center" RelativePanel.AlignHorizontalCenterWithPanel="True"/>
<Button x:Name="bt3" Margin="50,0,50,0" Height="100" Width="100" VerticalAlignment="Center" RelativePanel.RightOf="bt2" />
</RelativePanel>
</Canvas>
</Grid>

Answer 1

How to put those 3 buttons on the center of bottom?
Canvas is a layout panel that supports absolute positioning of child elements relative to the top left corner of the canvas.
<Canvas Width="640" Height="480" >
<Rectangle Canvas.Left="30" Canvas.Top="30"
Fill="Red" Width="200" Height="200" />
</Canvas>
For your requirement. Please use Grid panel to replace Canvas and set toolbarPanel VerticalAlignment HorizontalAlignment property like the follow.
<Grid x:Name="toolbarGrid" Grid.Row="2" Grid.Column="0" Grid.ColumnSpan="3" HorizontalAlignment="Center">
<RelativePanel x:Name="toolbarPanel" VerticalAlignment="Bottom" HorizontalAlignment="Center">
<Button x:Name="bt1" Margin="50,0,50,0" Height="100" Width="100" VerticalAlignment="Center" RelativePanel.LeftOf="bt2"/>
<Button x:Name="bt2" Margin="50,0,50,0" Height="100" Width="100" VerticalAlignment="Center" RelativePanel.AlignHorizontalCenterWithPanel="True"/>
<Button x:Name="bt3" Margin="50,0,50,0" Height="100" Width="100" VerticalAlignment="Center" RelativePanel.RightOf="bt2" />
</RelativePanel>
</Grid>