Fill all the screen with a stacklayout

Hello

As you can see in my image, I have Images (As buttons) the buttons are inside an horizontal layout, the width of the buttons is determined by the width of the screen size, since they are 3 the widht = screensize/3.

My concern here is that I have a main stack layout (vertical), but is not filling all my screen, and my buttons look terrible, How can I achieve this?

Here a code snippet

var screenSize = DependencyService.Get<IScreen>();

            double width = screenSize.Width;
            double height = screenSize.Height;

            StackLayout mainStackLayOut = new StackLayout{
                BackgroundColor = Color.Blue,
                HorizontalOptions = LayoutOptions.FillAndExpand,

                Orientation = StackOrientation.Vertical
            };

            StackLayout buttonsStackLayOut = new StackLayout
            {
                BackgroundColor = Color.White,
                //VerticalOptions = LayoutOptions.Fill,
                HorizontalOptions = LayoutOptions.Fill,
                Orientation = StackOrientation.Horizontal,
                Spacing = 0
            };
            mainStackLayOut.Children.Add(buttonsStackLayOut);

            Image doctorImage = new Image
            {
                WidthRequest = width / 3,
                HeightRequest = 50,
                BackgroundColor = Color.Gray,
                Source = ImageSource.FromFile ("about.png")

            };
            buttonsStackLayOut.Children.Add(doctorImage);

            Image patientImage = new Image
            {
                WidthRequest = width / 3,
                HeightRequest = 50,
                BackgroundColor = Color.Gray,
                Source = ImageSource.FromFile("twitternav.png")

            };
            buttonsStackLayOut.Children.Add(patientImage);

            Image sellerImage = new Image
            {
                WidthRequest = width / 3,
                HeightRequest = 50,
                BackgroundColor = Color.Gray,
                Source = ImageSource.FromFile("refresh.png")

            };
            sellerImage.GestureRecognizers.Add(profileTapRecognizer);
            buttonsStackLayOut.Children.Add(sellerImage);


            // Build the page.
            this.Content = mainStackLayOut;