What is the difference between studying social policy, international relations and politics in university?
This depends to some degree on how the various disciplines are set up where you are. In a typical US university, political science is a large discipline that includes international relations and (possibly) social policy within it.
Political science as a whole generally includes a variety of subfields. International relations is a major one of these. IR is the study of issues that transcend the politics of just one country. Political science departments often offer specializations in public policy studies. One aspect of this would be the study of policy that has to do with social issues.
The major difference, then, is that politics is the broad field while IR and social policy are smaller specializations within political science.