Rabbi Acha ben Jacob said: This expression, "three years old" (Gen. 15:9), is said only with reference to the mighty in power, as it is said, "And a threefold cord is not quickly broken" (Eccles. 4:12).
Rabbi Mesharshyah said: (Three years old) refers to a threefold (dominion) which they would exercise three times in the future in the land of Israel. At the first time each one would rule by himself; at the second time two together (would rule); on the third occasion (all) altogether to fight against the house of David, as it is said, "The kings of the earth set themselves, and the rulers take counsel together, against the Lord, and against his anointed" (Ps. 2:2).