Ant-Man: to get Ant-Man you have to do the 2nd mission that Lou Ferrigno gives you in Central Park in the New York hub.
Black Goliath: to get Black Goliath you have to destroy all 12 silver statues in the South Africa hub.
Fin Fang Foom: to get Fin Fang Foom you have to do a mission for him in Times Square in the New York hub.
Once you unlocked and bought all 3, hold down
so that you can turn big. Do that for all 3 characters in one sitting then you have your achievement.