I chose Door 1 since raging fire would not necessarily mean that the whole place is on fire. There may only be specific sections that may have active, intense flames, while other parts within the same area or perimeter are cooling down, smoking, or already burned out.
While Door 2 contains as dragon which is a mythical creature ( like Smaug or Vhagar) that could possibly survive without eating for 3 years and just hibernating and waiting to strike once you enter.
While Door 3 had an assassin who could have been freshly assigned there so instant death upon entry or he could have food supply from the inside that could have him last for 3 years.