The Royal Navy (RN) is the Senior Service, this is because it stayed loyal to the king during the English Civil War, and the army regiments all did their own thing. The Army is the middle service and the Royal Air Force being newest is the junior service.

The British Army is not called Royal because of the Civil war of not staying true to the crown, individual regiments were allowed to be called Royal and were given awards but not the army as a whole, and the Army is very attached to the difference between regiments, as an officer you are selected by individual regiments rather than the Army. The RAF and RN train each person who passes selection and it's not split like the Army by ship or naval base.

The Army has a land based role include supply and logistics as well as civil and mechanical engineering, intelligence and the usual armed roles (tanks, infantry etc) as well as the Army Air Corps (Flying Apache helicopters).

https://www.quora.com/What-is-the-difference-between-the-Royal-Air-Force-Royal-Navy-and-Army