Mike
I can't see any reason why not, although the LAA will need to be contacted about the MOD process, as I don't believe there are any in the UK with that gear on. Even if there is, a repeat MOD will need to be applied for. Not onerous, but worth mentioning.
Have you contacted the McBeans to ask them about it? John will give you an answer there and then over the phone.
The Mk 3 has the same number of lugs for gear mounting as the 4, so I can't see it would be an issue.
Out of interest, have you considered the Grove gear too?
I'm interested that Tom has managed to instal Grove gear onto a Mk2, as they don't have the mounting lug that's needed.