It would be up to the Organizing Authority or Race Committee per RRS 91 [The organizing authority or race committee shall appoint a protest committee.]

Technically, a protest committee can comprise just one person. There is no minimum qualification for that person. However, ideally you want a minimum of three people, each with a reasonable working knowledge of the rules. The chair should have some experience of chairing hearings, so that appeals on technicalities can be avoided. You haven't said how many clubs are involved, but a balance of PC members from each would be good, and if possible the Chair from a club that's not involved.

To answer the question posed by the original poster, anyone you like. There are no minimum requirements. The RRS only stipulate the composition of an International Jury. 

Also ... beyond the RRS, sometimes "conditions" are published for events, series or championships that stipulate minimum qualifications for race officials and jury compositions. 

Peter .. since this is a " interclub race series " .. are the clubs involved joint-OA's?  If so, you might want to float this back up the chain to the race-management folk at the clubs involved and have their joint organizing committee appoint the PC .. or provide you guidelines for doing so (which leans on Ant's point).

If you have one experienced judge (even if they are not "certified"), the other 2 can be experienced racers or people with solid rules knowledge.  If you end up with 2 strong panel members, then that provides an opp to bring along someone much less experienced but rules-interested.