I'm a graduate of the University of Glasgow. You're right in thinking that it is compulsory to study three subjects in your first year. I did Politics/Sociology/History.
I'm not sure what to suggest for you, just do what you think is the most interesting. If Economics is going to be the focus of your degree do something like Economic & Social History alongside it, with a little Politics maybe. It doesn't really matter, they're just filler courses anyway. Still have to get good grades in them though!