Try doing the following modifications. I have no idea if this works on WP7, though. I guess ActualWidth and ActualHeight should be considered, since you need to translate coordinates expressed in units to pixels.
private void iiiimage_Tap(object sender, System.Windows.Input.GestureEventArgs e)
{
Point point = e.GetPosition(iiiimage);
ApplicationTitle.Text = point.ToString();
var wb = new WriteableBitmap(iiiimage, null);
int x, y;
x = (int)(wb.PixelWidth * point.X / iiiimage.ActualWidth);
y = (int)(wb.PixelHeight * point.Y / iiiimage.ActualHeight);
int arrayPos = y * wb.PixelWidth + x;
PageTitle.Text = String.Format("({0} ; {1}) --> {2}", x, y, arrayPos);
int colorAsInt = wb.Pixels[arrayPos];
Color c = Color.FromArgb((byte)((colorAsInt >> 0x18) & 0xff),
(byte)((colorAsInt >> 0x10) & 0xff),
(byte)((colorAsInt >> 8) & 0xff),
(byte)(colorAsInt & 0xff));
r3ct.Fill = new SolidColorBrush(c);
}