The line from the summary that best justifies the conclusion that "Harry Potter and the Sorcerer’s Stone" was influenced by Ancient Greek mythological literature is:
"To help save the day, Harry must get past a scary three-headed guard dog."
This line refers to a three-headed dog, which is reminiscent of Cerberus, the mythological creature from Greek mythology that guards the gates of the Underworld. Such a parallel suggests an influence from ancient myths in the story.