These are only a few chord combinations. There are many more which start with 'sharps' and 'flats'. The same concept is to be used with songs starting with minor chords. Few of such chord combinations of these songs are Am - F - G, Bm - G - A, Em - C - D, and many others. An example can help you better understand. Suppose a song starts in the 'G' scale, if you are aware that G scale comprises the relative chords C and D, you can easily play the song.