According to the Feds the US prison pop. (Fed and state) is about 1.5 million total. Let's say the county jails gets us to 2 million total in the us. Whats the % of foreign nationals in that total? 7%, 8%? That only gets you to 160,000. And if we pull and deport them that most likely means they get freedom in another country. Even triple that number to account for prisoners who served and were released won't even get us to 500,000. I just don't see where the 2-3 million number comes from.
There are plenty who have been convicted of crimes but are already out and were not deported as per the law.