First was Lt. General Michael Flynn. He lasted 3 weeks. He was indicted for lying to the FBI about contacts with Russians. He plead guilty. Trump pardoned him.
Second was Lt. General HR McMaster. Trump fired him after 11 months. He described the Trump White House as competitive sycophancy. He also said Trump wants flattery and is addicted to adulation. See video
His third was John Bolton. Bolton served for 17 months and has said Trump should never be President again. He essentially called Trump a buffoon in this interview.