Institutions, Systems, Structures

Folks who work on oppression often distinguish oppression attributable to individuals from oppression attributable to institutions.  Thus, there's a lot of discussion about institutional racism or sexism, say, as well as discussion of systematic or structural racism or sexism.  Here's a quick question: anyone have any thoughts on the nature of the relation between the institutional, the systematic, and the structural?  Are these terms just being used as synonyms, at least in the relevant literature?  I have a vague feeling that it is possible to have non-systematic and non-structural institutions, but (since the relevant systems and structures are all social systems and structures, I presume), I'm not sure that there are any non-institutional (social) structures or (social) systems.  Any thoughts?