Abraham and Sarah still had not had Isaac, but God is talking about many nations in the past tense. He talks of things that are not as if they already are.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 27)
God made a twofold promise to Abraham. The first was a material promise that he would be the father of many nations and that kings would descend from him. God promised him that his progeny would inherit the land of Canaan, an expanse that He defined as stretching from the Nile to the Euphrates rivers. The second, but more important, promise was spiritual. God promised Abraham that in his Seed all the nations of the earth would be blessed. This promise encompasses the life and work of Abraham's best known and most revered descendant, Jesus Christ.
This promise was later extended to include the inheritance of the whole world (Romans 4:13). Abraham's physical descendants, the nation of Israel, inherited the land of Canaan. This was a type of Abraham's spiritual descendants inheriting the earth.
Earl L. Henn (1934-1997)
Basic Doctrines: The Reward of the Saved
Other Forerunner Commentary entries containing Genesis 17:3: