It somewhat depends where you're applying and for what subject. Many good RG unis would consider you for many of their subjects outside of the elite few; Southampton, Exeter, Loughborough, and Manchester come to mind.
Also different unis have different metrics for what they consider a facilitating/preferred subject. LSE, for example, considers both Sociology and Psychology preferred, and since you've taken two preferred subjects in theory the third non-preferred would be acceptable. However it depends what course you apply to; some are more competitive than others, and for something such as Law you would probably find it difficult to get an offer. For a slightly less competitive course such as Social Policy or Anthropology perhaps, you may have better luck.