The Sonett III came about because some equipment that would be required for upcoming US emissions laws required changes to the structure of the Sonnet II.  It was somewhat more attractive than the II and subsequently slightly more popular. 

It was introduced in 1970 and initially had the same V4 engine as the Saab 96.  Demands upon its sporting nature combined with the increasing performance toll of emissions equipment required the inststallation of a bigger 1.7 liter V4 midway through 1971, although horspower stayed the same.  A total of 8,368 were built before slowing sales brought about its cancellation after the 1974 model year.  This was eight times what the Sonett II had achieved, but they had been hoping for more acceptance.  It was still a Saab though, and the marque had not yet attained a large degree of cachet with the general public.  Although they were increasingly poorly made by that time, Jaguar, Triumph and MG were more recognizable to potential sports car buyers.  The Datsun 240Z also had a large following.