Gen 6:3 clearly refers to 120 years before God would send the flood that would destroy mankind.
Noah found favor in God's eyes, and fathered Shem, Ham and Japheth (Gen 6:8-10).
Noah was 600 when the flood came (Gen 7:11).
Others lived beyond 120 years who were born after the flood - Abraham 175, Isaac 180, and Jacob 147.
For to me, to live is Christ and to die is gain. (Phil 1:21)
Blessings