As Felljunior and I have been saying - the idea in principal sounds like fun - the idea in practice is a non-starter.

1. If you make it single age group, hardly any club will get a team out - who has 3 runners in an age group?
I cannot genuinely do it in age age group, but could draft in track runners to make up most age groups.
Genuine Fell runners, perhaps 4 clubs may get a couple of teams out.

2. If you make it mixed age group, you then have perhaps 6 clubs max that can get a mixed age group team together from genuine fell runners.

The only option is to remove the club element. It works for the Bunny Run relays. Allow athletes to come together (or be drawn together) to make relay teams of 3.

The only problem with this option is that it then becomes less of a competitive event and more a fun event, and people may be less inclined to travel for it, or take time out to do it when there are other "real" events around.