Think carefully!!!! Can you confine the w/ex to a few months eg Feb- May? My concern is the workload, when you are already working P/T (and good for you for doing so).
Unless this is volunteering in an oral surgery/ other dentally specific area, you may be better using reflections on the transferable skills from your part-time work (eg people skills, team work, balancing time apply to almost everything ) and spending those 3 hours a week ensuring you get your predictions a great UCAT score, and most importantly getting a rest or downtime.