Kate has three African snails named Sluggo, Pokey, and Lag. Each of the snails loves to eat lettuce. One day, Kate gave each snail the same number of heads of lettuce. The snails started munching away. Each snail ate at a constant rate of speed, but they ate at different rates of speed from each other. When Sluggo finished all of his heads of lettuce, Pokey had 17 heads left and Lag had 26 heads left. When Pokey finished all of his lettuce, Lag had 12 heads left. How many heads of lettuce did each snail start with?