The correct answer is true.
It is true that when the war ended, approximately 3 million American men had served in the military on both sides.
We are talking about the Civil War in the United States, which started in 1861 and ended in 1865. Yes, approximately 3 million American men served in the military on the Union Army side or the Confederate Army side. Many reasons caused the internal confrontation of Americans, but the most important one was the issue of slavery and abolitionism. The Northerners supported abolitionism and the Southerners supported slavery because they depended on slaves to produce cash crops in the large plantations.