Sixteen aircraft had been delivered by the end of 1952, but the problems suffered by the type stalled further sales for several years. The last four of the twenty Drovers built were produced in 1953 but were not sold until 1955 and 1956. In another bid to rectify the type's poor performance DHA re-engined seven Mk. 2 aircraft with Lycoming O-360 horizontally-opposed engines driving Hartzell feathering constant-speed propellers. Changes were also made to the flap control system and the tail wheel assembly. The first modified aircraft, re-designated as a DHA-3 Mk. 3, was returned to its owner the Royal Flying Doctor Service of Australia (RFDS) on 4 June 1960. Three Mk. 3s were later further modified; two aircraft operated by the NSW Section of the RFDS were modified in 1962 as the Mk. 3a with the tailplane altered to have seven degrees of dihedral (14° according to one source) and the span increased by 2 ft (61 cm). The third was modified as a Mk. 3b with an increase in MTOW of 300 lb (137 kg) to 6,800 lb (3,087 kg).