I just read an article that says this number came from a report that discover three million legal residents didn't understand the question correctly and listed themselves improperly as a "non resident".
While we have no idea how Phillips arrived at his claim that 3-million noncitizens voted, people who have made similar claims in the past have cited a 2014 report that claims 6.4 percent of non-citizens voted in 2008 and 2.2 percent of non-citizens voted in 2010 midterm congressional elections.
That report was based on data from a Harvard survey of people. But the data was flawed, which created flaws in the subsequent report.
The authors of the survey say a small percentage of respondents, who are citizens, accidentally misidentified themselves as noncitizens on the survey. This is because the respondents didn?t read the question carefully and accidentally selected the wrong response to the question.
Fact-check: Did 3 million undocumented immigrants vote in this year's election? | PunditFact
It's stunning that the President of the United States spits out facts and figures without researching them first. He has the entire United States Government at his disposal - I am confident someone at the White House can research this crap for him.