Frequency Spectrum of an image
You only get one row of three dots because the image is constant in the vertical direction.  That means that vertically speaking, the image only has 0 Hz content.  Thus, you get the three dots in row 0 (the 0 Hz row), and nothing in any of the others.  If you made one of the image's rows different than the others then you would see differences in the FFT's rows as well.