Albert Barnes Commentary Deuteronomy 5:3

"Jehovah made not this covenant with our fathers, but with us, even us, who are all of us here alive this day." — Deuteronomy 5:3 (ASV)

The “fathers” are, as in (Deuteronomy 4:37), the patriarchs, Abraham, Isaac, and Jacob. With them God did indeed make a covenant, but not the particular covenant now in question. The responsibilities of this later covenant, made at Sinai by the nation as a nation, attached in their day and generation to those whom Moses was addressing.