My only suggestion is that there should be a Joint officer cadet training establishment.
A seriously bad idea that should never be allowed to happen. Each service has a distinct ethos & requirement so it would not be possible to blend the three services together & if you tried it would potentially lead to amalgamation which has already been proved not to work - disaster all round.
As the largest service the army would expect to take the lead so the course would most likely churn out infantry officers which would not meet the requirements of the RAF or RN. There could be a 'joint' ('Joint' in army-speak = 'army') first term before the 3 services split for more focused training but that would negate much of the potential savings. The graduation parade would look messy with the 3 differing uniforms or would everyone wear the same (army) uniform?
At the start of your career you need to be immersed in the culture & traditions of your own service, a course of 70% army/ 15% RN & 15% RAF (my guesstimate) would not achieve that for the smaller services.