For Psychology at Bristol - as just one example - you need to be doing one 'science related subject', and they include Geography, Computer Science and Psychology on that list. There will be many other courses/Unis who will not want Biology and will be quite happy with just Psychology.
Sociology is not considered a soft subject - its a solid, academic, essay-based subject.
It would compliment Psych well. And if the 3rd subject is nt specified by Unis, then it makes sense to take a subject which you feel you will enjoy and are likely to get a good grade.