Here's the situation: I want to bring my girlfriend over here to Portugal for the holidays. She's Venezuelan but is in Trinidad and Tobago right now. My plan is to save money by buying 2 different plane tickets, one from Trinidad to Miami, and another one from Miami to Portugal. My question is, does she need a visa to stay in the US for 3 hours to catch another flight?
Yes, she needs a C-1 visa to change planes in the US, because the US has no exit border controls, so domestic and international flights aren't segregated in the departures area. In addition, it is possible to exit the departures area without any checks.
Thus, anyone touching down at a US airport must enter the US.
This fact is stated in Timatic, the database used by Airlines.
There are no TWOV facilities available in the USA
(TWOV=transit without visa)