But this was no problem for Herakles. He just shot one arrow from the side and it went through all three of Geryon’s bodies. You might compare this story to the Bible story of David and Goliath, maybe.
Twelve Labors of Hercules (Step into Reading, Step 3), by Marc Cerasini. Very easy, for beginning readers.
The Story of Hercules (Dover Children’s Thrift Classics), by Robert Blaisdell (1997). Easy reading, very cheap.
Hercules, by Nancy Loewen (1999). More sophisticated, with a look at how the myth was passed on and what it meant to people, as well as the story itself.
D’aulaire’s Book of Greek Myths, by Edgar and Ingri D’Aulaire. (Look under Heracles).
The Myths of Herakles in Ancient Greece, by Mark W. Padilla (1998). By a specialist; serious stuff.