All three of Mr. Reagan's latest Cabinet nominees are highly competent people of the sort described, in this administration's vocabulary, as realists. The term means to say that they are reasonable men who are not greatly preoccupied with ideological warfare. But as Cabinet officers, two of them will be limited to one degree or another by the revival of the president's inclination to abolish their departments.

The White House has been thinking of pushing the disassembled pieces of the Energy Department into the Interior Department. The present secretary of energy, Don Hodel, was undersecretary of the interior before he moved to his present job, and knows both departments better and in greater detail than anyone else in the administration. If the two departments are to be amalgamated, Mr. Hodel is the obvious choice to carry it out.

John Herrington, assistant to the president for personnel, is to replace Mr. Hodel at Energy. Temporarily? Perhaps. Then again, it's possible that Congress will be no more receptive to the demolition of the department in Mr. Reagan's second term than it was in his first. There are good and urgent reasons to retain a full-fledged department to manage the many federal responsibilities in energy. In that case, Mr. Herrington's lack of experience in the infinitely complex subject of energy economics may become more important than Mr. Reagan hopes or expects. Mr. Herrington has the reputation of being a very fast learner, and that is a good thing: the kind of international events that disrupt the life of a secretary of energy require plenty of knowledge and skill.

To the Education Department, William J. Bennett brings a passion for culture in the genuine sense of the word. The job of a secretary of education, in this administration, is to fend off the numerous people who want to use federal intervention to advance their various social causes, and instead to keep attention fixed on the improvement of teaching and learning. Mr. Bennett's predecessor, T. H. Bell, accomplished that feat with great but unobtrusive skill. Mr. Bennett is likely to make more noise. But if he follows the track that he set for himself at the Endowment for the Humanities, it will be the right kind of noise in behalf of the classic idea of intellectual quality. Perhaps he will want to give some thought -- publicly -- to the condition of the colleges and universities, a subject on which he is well informed.

Whether the Education Department survives or vanishes is not a matter to lose sleep over. The case for establishing it was weak. But dismantling it is not worth a long fight distracting the attention of Mr. Bennett, and others who care about education, from more substantial concerns.