I believe you only need to have your G3 unvented cylinder qualification for standard mono heat pumps.
Mono heat pumps have a sealed refrigeration system and so no F gas work is required.
Hydro split ASHP's use water between the outdoor unit and indoor unit so no F gas required.
A refrigerent split does however require F-Gas qualification as refrigerent pipework is required between the outdoor unit and indoor unit, benefit is the outdoor unit can be sited much further away.
MCS (and umbrella schemes) are only required at present if you are claiming for the RHI, new build etc does not require MCS.
The majority of installs are mono systems using the likes of Daikin, Samsung, Mitsi etc and so would only require G3 unvented certificate. (Plus MCS if claiming RHI).