According to Wikipedia (a reasonably impartial source) NAFTA dropped tariffs between the three countries to zip in nearly all cases by now. It took President Bill Clinton's best efforts to get NAFTA ratified over the dead bodies of US unions. NAFTA did increase trade between Mexico, Canada and the US by a lot, perhaps 50% over the years since 1993 when NAFTA was ratified. It also did contribute to US job losses of maybe 500,000 jobs. These numbers can be controversial, but Wikipedia is the most balanced source I am aware of.
We had The Donald on TV yesterday trashing NAFTA up one side and down the other. He promises to "renegotiate" the NAFTA treaty. He claimed that NAFTA is a US job killer. In this, he has, or ought to have, the warm support of US unions who have been anti NAFTA since the beginning.
We had the "three amigos) (Obama, Trudeau, and I can't remember the name of the Mexican president) on TV today. All saying nice things about NAFTA, and the need to keep it going.
Nobody said anything about admitting the UK to NAFTA.