I examined the Hirth Trio prototype - at Hemet, California - circa 1996.
It was one of a short-lived generation of three-surface, experimental airplanes.
It’s nearest competitor was the Australian Eagle rainer started by Burt Rutan. After later investors changed the project, Rutan dis-owned the Eagle project. Aerodynamicist John Roncz helped redesign the Eagle.
Tri-surface airplanes were an out-growth of the generation of light canards designed by Burt Rutan during the1970s.
The primary goal of tri-surface airplanes was to displace the main wing away from the cockpit, either to ease entry (Hirt) or simplify the cabin pressure vessel.
The only tri-surface that made it into production is the Piaggio Avanti light business turboprop. Designers wanted a simple, straight wing and a simple cabin pressure vessel, so they routed main wing spars aft of the cabin. When this unbalanced the airplane (nose-heavy), Piaggio engineers added an extra lifting surface to the nose. This canard can be considered an extension of the main wing because it has no control surfaces, similar to Rutan’s Catbird.
Hirt used tri-surfaces for similar reasons in that he wanted to route the main spar aft of the cockpit and wanted easy access to the cockpit.
Hirt only built one prototype and we have not heard from it since the 1990s.