If you don't see the UC-eLinks link in your results, click on Settings in the upper right, then Library Links and select University of California, Los Angeles as your institution.
All three have search boxes in the first column of this page.
Major online database vendors like ProQuest and Ebsco host many disparate databases, but you can use their search engines to search all the databases from that vendor at once. This technique also works for some major journal and e-book publishers that have search engines which work across all of their titles.
Search platform for over 120 databases, from the AFI Catalog to Worldwide Political Science Abstracts. Includes scholarly articles, primarily in the humanities and social sciences, as well as newspapers and dissertations.
Search platform for over 50 databases, from Academic Search Complete to Women's Studies International.
Fifteen databases, plus individual journals and e-books. Includes 2011-present IEEE-Wiley books.
Immense array of books, chapters, conference proceedings and journal articles in the social sciences and sciences, published by Springer-Verlag. After searching, unclick Preview Only Content, whose materials are not under UCLA subscription. Advanced search enabled at wheel icon to right of search.
While searching multiple databases at once can be useful, by definition it's lowest-common denominator searching. None of the advanced search techniques available in individual databases will be available. Some pitfalls to watch out for:
For more info about the quest for a single search engine to find everything, see...