Your code will try to play song #3 every second if you hold that button down. You should look at the State Change Detection example in the IDE (File->examples->02.digital->State Change Detection) to learn how to detect when a button is pressed, not if a button is pressed.
Also, your waitMilliseconds() function is the same as delay() since the myDFPlayer.enableLoop() is commented out. I think that was the whole point of the function.