<Grid>
 <Grid.RowDefinitions>
  <RowDefinition Height="Auto" />
  <RowDefinition />
 </Grid.RowDefinitions>
 <Grid>
  <Grid.ColumnDefinitions>
   <ColumnDefinition />
   <ColumnDefinition Width="Auto" />
   <ColumnDefinition Width="Auto" />
   <ColumnDefinition Width="Auto" />
  </Grid.ColumnDefinitions>
  <Button Grid.Column="1" Content="Кнопка 1" Visibility="???" IsEnabled="???" Tag="1" />
  <Button Grid.Column="2" Content="Кнопка 2" Visibility="???" IsEnabled="???" Tag="2" />
  <Button Grid.Column="3" Content="Кнопка 3" Visibility="???" IsEnabled="???" Tag="3" />
 </Grid>
<!--... тут ContentControl, отображающий по требованию UserControls -->
</Grid>

this.mainWindow.GetElementByName(string elementName)

((TextBox)GetElementByName(@"usernameTextBox")).Text = @"admin";