Reference is from interviews, alluded to on his policy page "A Universal Basic Income at this level would permanently grow the economy by 12.56 to 13.10 percent—or about $2.5 trillion by 2025—and it would increase the labor force by 4.5 to 4.7 million people. Putting money into people’s hands and keeping it there would be a perpetual boost and support to job growth and the economy."
Saying UBI would grow the economy is not the same thing as saying that UBI growth would pay for UBI. My understanding is that Yang doesn't include it in the 3tn - most comes from replacing existing welfare programs and VAT.