car has lil heat

ok so i turn on the heat well after the car is warmed up the heat stays on for about 3 minutes then cuts to cold air the car is not over heating the tempature gauge is right where its suppose to be i was told like 5 different things from mechanicz and now i dont know what to do i drive a good 40 miles one way to work a day and dont want to freeze my butt off anymore...

could be bad temp sensor, gauge, or sounds like blend door not working or heater core plugged up