Tem que definir em qual linha fica cada elemento
<Grid>
<Grid.RowDefinitions>
<RowDefinition/>
<RowDefinition/>
<RowDefinition/>
</Grid.RowDefinitions>
<Label Grid.Row="0" Text="Brasil"/>
<Label Grid.Row="1" Text="Argentina"/>
<Label Grid.Row="2" Text="Uruguai"/>
</Grid>
