HorizontalAlignment
solve your problem. But parent control must set width
<Button Height="55">
<StackPanel Orientation="Vertical" Width="180">
<TextBlock Text="{Binding Item.Quantity}" HorizontalAlignment="Left"/>
<TextBlock Text="{Binding Item.ItemName}" HorizontalAlignment="Center"/>
<TextBlock Text="{Binding Item.ItemSpecification}" HorizontalAlignment="Center"/>
</StackPanel>
</Button>