Proquest and EBSCO's Education Journals index scholarly journal articles, there is some overlap but each database has some journals the other does not. Even when they have the same journal they may have different issues available as full text.
Much educational theory is application of psychology research on how humans learn, so Psychology and Behavioral science is included as well.
ERIC is a little different. Most ERIC documents are available as full text, but you don't need a subscription to access them. ERIC is funded by the Department of Education, so it's open to the public.
