DEN - Denver International Airport, Colorado, USA (as Frontier Airlines is based in Denver)
MCO - Orlando International Airport, Florida, USA (as the text mentions a one-day sale)
LAS - McCarran International Airport, Las Vegas, Nevada, USA (as the text mentions a Vegas sale)
Please note that these are just guesses based on the information provided, and it's possible that the text does not actually refer to airfare deals or specific airports.