I doubt that Van's will have anything off the shelf to fit the RV 3. Some builders have added a 5th point mount to the F 304 bulkhead (which forms part of the control mixer support structure), this puts it further forward than is ideal and could, (I think), lessen it's effectiveness. The RV 3 also has a shorter seat rib right on the center line of the airframe so any sort between the seat ribs fixture would have the mount point slightly off center.
The plans also have the seat pan permanently fixed with blind rivets so unless the builder installed nutplates/screws instead you would need to drill those out to gain access to the seat ribs.
A 5th point belt mount is doable but you will most likely have to put your ingenuity to work.
Russell