Joint officer training has been mooted for years now. However, I know that Dartmouth is not actually owned by the Navy, but granted to them for use as a training establishment by a local landowner (historic thing...!) hence it cannot be 'sold' by the Navy as they don't technically own it.
Not sure if there is anything similar with Cranwell or Sandhurst, nor whether they'd even be big enough to accommodate all 3 services...?
The difficulty is that none of the services would want to get rid of their iconic establishments and move to someone else's, so perhaps the only answer (if there were to be a joint location) would be to get rid of all 3 and build a bespoke establishment somewhere?