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In classical mythology, a large, hollow horse made of wood used by the Greeks to win the Trojan War. The resourceful Odysseus had come up with the plan for the horse. The Greeks hid soldiers inside it and left it outside the gates of Troy. They anchored their ships just out of sight of Troy and left a man behind to say that the goddess Athena would be pleased if the Trojans brought the horse inside the city and honored it. The Trojans took the bait, against the advice of Cassandra and Laocoon. That night the Greek army returned to Troy. The men inside the horse emerged and opened the city gates for their companions. The Greeks sacked the city, thus winning the war.
Note: The story of the Trojan horse is the source of the saying “Beware of Greeks bearing gifts.”
A three-part attack, difficulty, threat, etc: Triple Whammy on the Farm (1940s+)