They are different species with different appearances from differing regions.
All are of the genus Testudo, but Testudo graeca have little spurs or tubercles on the top of the inner thigh (hence spur thighed), Testdo hermanni do not have these spurs but do have a spur/claw at the end of the tail and a different head shape amongst other less obvious things, Testudo horsfieldii have neither of these characteristics and are rounder and not so domed, being a burrowing species and needing the round shape to turn around easily in their burrows.
Hope this helps