Good question. Number 1: Mr. Trump has said that he plans to deport as many as 3 million immigrants with criminal records. Currently, there are only about 850,000 illegal immigrants with criminal records. This means that if his plan were implemented to its fullest extent, there would be 2,150,000 people deported who:
1.) Are resident aliens with previous criminal records, but have green cards, workers' visas, and the like. No matter what their criminal records, they cannot be lawfully deported as long as these documents are valid and they abide by any residency requirements contained within them.
2.) Entered the United States illegally, but have no criminal record. Your characterization of illegal immigrants as "criminals" is incorrect. In the context of the American legal system, entering the country without authorization is a civil infraction (similar to a speeding or littering ticket), not a criminal offense. Although the law does make provisions for deportation, it's important to know that our current immigration courts are backlogged with record-breaking numbers of legal, valid deportation cases pursued by President Obama's administration.
To make a long story short: Mr. Trump's plan is completely full of hot air. It is not legal or practical to implement to the extent that he has suggested.