The first three chords for "Free Fallin'" are among the most recognizable song openings in rock music.
Petty originally was noodling around with a more complex guitar part, but during a jam session, fellow musician Jeff Lynne told him to just repeat those first three chords over and over instead of trying to do something more complicated, and the song worked perfectly.
Many artists fill their songs with minor keys, complicated progressions, discordant notes, key changes, and the like, but there's something to be said for the rare song that draws its sound from three simple, clean chords. The chords used in "Free Fallin'" are literally repeated over and over and over again in the song in both the verses and the chorus (though the chorus jumps up an octave and is higher-pitched, which adds a bit of difference).
Maybe you're picking up an instrument for the first time and are wondering if you can make great music with just a simple chord progression. Tom Petty's "Free Fallin'" proves that you most certainly can.