Answer:
Those would be Guerrero, Oaxaca and Chiapas.
See, Mexico is a federal republic composed of many independent states, just like the US. Those three states are what is commonly known in Mexico as "the south" or "the southern states".
http://en.wikipedia.org/wiki/Guerrero
http://en.wikipedia.org/wiki/Oaxaca
http://en.wikipedia.org/wiki/Chiapas