uniformgridfix.xaml

<Window x:Class="wpf_test.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:wpf_test"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <StackPanel>
        <UniformGrid Columns="3">
            <Grid
                Background="Gray"
                Margin="5"
                VerticalAlignment="Stretch">
                <Grid
                    Height="100"
                    Width="100"
                    Background="Black"/>
            </Grid>
            <Grid
                Background="Gray"
                Margin="5"
                VerticalAlignment="Stretch">
                <Grid
                    Height="100"
                    Width="100"
                    Background="Black"/>
            </Grid>
            <Grid
                Background="Gray"
                Margin="5"
                VerticalAlignment="Stretch">
                <Grid
                    Height="100"
                    Width="100"
                    Background="Black"/>
            </Grid>
        </UniformGrid>
        <UniformGrid Columns="3">
            <Grid
                Background="Gray"
                Margin="5"
                VerticalAlignment="Stretch">
                <Grid
                    Height="50"
                    Width="100"
                    Background="Black"/>
            </Grid>
            <Grid
                Background="Gray"
                Margin="5"
                VerticalAlignment="Stretch">
                <Grid
                    Height="250"
                    Width="100"
                    Background="Black"/>
            </Grid>
            <Grid
                Background="Gray"
                Margin="5"
                VerticalAlignment="Stretch">
                <Grid
                    Height="100"
                    Width="100"
                    Background="Black"/>
            </Grid>
        </UniformGrid>
    </StackPanel>
</Window>