Three-Year Border and DACA Extension Act This bill: (1) authorizes funding in FY2018-FY2020 for improving border security; (2) establishes a Border Security Enforcement Fund to provide for border security-related costs; and (3) requires transfers from the fund to construct, replace, or plan physical barriers along the border. The bill directs the Department of Homeland Security to grant three-year (beginning after the enactment of this bill) provisional protected presence and employment authorization to an alien who: (1) was born after June 15, 1981, entered the United States before age 16, and has continuously resided in the United States since June 15, 2007; (2) was physically but unlawfully present in the United States on June 15, 2012; (3) has met certain educational requirements or has been honorably discharged from military service; (4) has not been convicted of a felony or certain misdemeanors; and (5) does not pose a threat to national security or public safety. An alien who is in the deferred action for childhood arrivals program (DACA) is deemed to have protected presence through the expiration date of his or her deferred action status.