I wouldn't go with the RA advisor. I think his recommendation could marginally suggest that you're a decent person, but I doubt he'll be able to really speak to your ability to succeed as a law student. The rule of thumb is always to error towards academic sources.
That said, you don't want to be redundant by submitting 3 letters of rec which pretty much say the same thing. Schools will only spend so much time reading your app, so an extra rec which adds nothing new will only take away from other more unique and original portions of your application.
I think the head of the polisci dept could be helpful considering that this individual knows something about your academic promise and ability. Since you're on good terms with this person and have not taken a class with this professor, one can infer that this person will write about more subjective qualities which won't appear in your other two LORs. I think the important part is that this recommender can also mention and discuss at least in part, your intellectual capacity. If this recommender is able to add new insights into you as a person while still speaking of your ability to succeed in law school, I would think that this is the route to go.