Keep in mind that at that management level people are seldom outright "fired", either in government or in business, as it shows that the person doing the initial hiring made a keen error in judgment. It's usually done by "negotiated" resignation. The general public is unaware of the true reason for the departure(s). (Apparently Mr. Trump doesn't concern himself with such "niceties".)
But let's start with the Secretary of Defense. First there was Robert Gates, then Leon Panetta, followed by Chuck Hagel and finally Ashton Carter (what was up with that?). For Secretary of State Hillary was "replaced" by John "gigolo" Kerry. Steven Chu unceremoniously departed as Secretary of Energy to be replaced by Ernest Moniz. Secretary of Health and Human Services Kathleen Sebelius was shown the door after some controversy and replaced by Sylvia Mathews Burwell. And Secretary of Education Arne Duncan "resigned" and was replaced by John King, Jr.