Typically the Tri-Service concept is the removal of duplication and the standardisation of procedures - but also saves HMG some folding.
A standard phrase to describe Tri-Service activities is 'purple' - the innocent kevlar hat therefore becoming the 'purple helmet' (also used to describe that part of a soldier used in the process of creating pad brats in partnership with a Ron Hill-wearing slapper.
Tri-Service shortcuts are generally made up by a Civil Servant as being ' a good idea, and useful to all three services'. In practice it's generally a crap plan and usable to nobody. A shortcut to FUBAR.