Help pick a degreeWatch
HSPS at Cambridge lets you study all three, as in first year you can take one paper in each of those areas, and from second year you can specialise in politics & sociology (the politics side includes IR papers I believe). Also any joint course in politics at Oxford allows you take as part of the politics half of your course some papers in sociology.
Beyond that, you might also be interested in courses in social policy, which will probably include aspects of both sociology and politics (and for international social policy i.e. at LSE, some elements of IR maybe). Note this is not in the sense of taking discrete papers/modules in each of those separate subjects, but in the sense that the field of (international) social policy is informed by them so your social policy studies will have elements of those areas coming up in the process.
Social anthropology also has some elements of sociology (in terms of social theory) and a lot of the issues considered are closely entwined with political considerations or may relate to aspects of modern social/political theory (e.g. various methods of social control including colonialism and cultural and social structures which in the modern era often tie into political structures in some regions). You can take social anthropology papers in HSPS at Cambridge too
From what I've heard Bristol has great student life and is an overall good university.
Edit: Similar universities include Exeter, Birmingham, Sheffield, etc.
Also, as previously mentioned, there will be a lot of overlap regardless so it would be hard to do one without the other. Plus, for example, if you did politics and sociology (or either singly) but found your passion was for international relations, you could easily do an international relations masters or PhD with that degree.