United Airlines is operating from Terminal 3 within the San Francisco International Airport, or SFO. United Airlines SFO Terminal This modern terminal contains food, retail, and comfortable seating areas. The check-in counters of United's flights, security checkpoints, and gates are located in Terminal 3, making this the main base for both its domestic and international flights. It is connected to the AirTrain system and offers a fast way of connecting to other terminals, parking, and rental car facilities. The airport experience for travelers using United at SFO is thus convenient and efficient.