The H.III series was an incremental development of the Horten H.II with reduced sweepback of 23°, span increased to 20 m (65  ft 7 in) and modified lateral controls.

In high-speed flight, the nose down trim was provided mainly by the inner elevon section moving downwards, the outer flap deflecting only slightly; this had the advantage of relieving the tips of torsional loads at high speed.

Subsequent H.IIIs were built at various locations including Peschke Flugzeugban in Berlin, Fürth, Giebelstadt, Minden, Bonn and Göttingen.

Built specifically for the 1938 Rhön competitions the H.IIIa was found to have unsatisfactory turning performance, so the H.IIIc, to be flown by Werner Blech, was modified with a canard surface mounted above and in front of the cockpit to assist with pitch control at low speeds.

[2] Blech warned the other pilots not to follow him into the same cloud and took an aero-tow from Walter Horten in their Focke-Wulf Fw 56 glider tug.

For various reasons the H.IIIs had a lacklustre showing, with a best placing of twenty-second by Scheidhauer, (partly due to his retrieve crew having been detained by customs for four days at the Czechoslovak border, on return from a competition flight).