Combine first 3 ingredients in a medium saucepan; bring to a boil. Place masa harina in a large bowl; add boiling mixture, stirring with a wooden spoon. Cover; let stand 15 minutes.
Divide the dough into 12 equal portions. Working with 1 portion at a time, shape each portion into a ball with moist hands. Shape the balls into 1/2-inch-thick patties. Coat each patty with cooking spray.
Heat a 10-inch cast-iron skillet over medium heat until hot. Place 4 arepas, cooking spray sides down, in pan. Press arepas lightly with a spatula to flatten slightly; cook 5 minutes on each side or until crispy and speckled brown. Repeat with remaining arepas.