Genesis 3 is titled in most Bibles as "The Fall"--and rightly so.
In Genesis 3 we see the terrible scene in which sin and death entered the world. But we also see the first prophecy of the defeat of sin and death and salvation through the Lord Jesus Christ. When addressing the serpent, God said, "I will put enmity between you and the woman, and between your offspring and her offspring; he shall bruise your head, and you shall bruise his heal" (Gen. 3:15).
If you choose to preach the Law and the Gospel after reading Genesis 3 in the open-air, there are several transitions you can use.
1) Original Sin -- Talk to the crowd about how every man, woman, and child is born with a sin nature.
2) The Human Conscience -- "Then the Lord God said, 'Behold, the man has become like one of us in knowing good and evil'" (Gen. 3:22a). Talk to the crowd about how God has given every person a conscience, writing His Holy Law on their hearts.
3) The Cross -- As previously mentioned, there is a Messianic prophecy in this chapter (Gen. 3:15). It can serve as a wonderful springboard for the cross of Christ and his defeat of sin, death, and Satan.
I hope these suggestions are helpful to you. May the Lord bless your efforts this weekend as you read His Word, in the open-air.