What terminal is Frontier Airlines at PHX? The majority of Frontier Airlines' domestic flights are based out of Phoenix Sky Harbor International Airport's (PHX) Terminal 3. Travelers can easily navigate Terminal 3, which provides simple access to security checks, baggage claim, and ticketing. Check-in is on Level 2 close to the British Airways counter at Terminal 4, which Frontier utilizes for some international routes, including flights to Cabo San Lucas. For a seamless check-in and boarding process, passengers are advised to arrive at least two hours before departure for domestic flights and three hours prior to departure for international flights.