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 Indians?

The Native Americans, or as you put American Indians, died for a number of reasons. One of those reasons was because the European settlers that colonized America brought over many diseases that the Native American's were not equipped to combat, diseases like measles. Many of them also died while moving westwards after being kicked out of their home lands, and others died while fighting in wars for their territory. The only American Indians who would be left would have been on reservations at the time.