Period 1 begins with 1491. if the american indian population in
what is now the united states was nearly 10 million before 1492,
why is the united states population in modern times only 2 to 3%
american indian?

That is because the vast majority of Native Americans died in the period. This was mostly due to diseases that the Europeans brought with them and against which they didn't have the immunity. Many also died in wars and in reservations, but mostly due to diseases. The Europeans flourished and developed successful colonies, which led to the modern times.