Starting my third year of University, how do I decide if the army is for me, and between the regulars or reserves?

One of the best things to do at university is join your local UOTC if you haven' already. This will give you a good insight into whether the military is a career path you should pursue. It will give you the opportunity to visit military units and also expose you to the world of adventure training. As for whether you should join the reserves or regular this is very much dependant on whether you have a different career path you wish to pursue which the military could complement. For myself, I made the decision to join as a regular because the experience will ensure I am able to get my CEng by my 8th year in the job. When/If I decide to leave the army, I know that I will have a significant amount of desirable experience to move into industry.

Hope that helps

UOTC isn't typically available to students in their third/final year at University.