If you want a fully qualified GP-track resident, there's probably no way to compress the learning process much further down than the four years of American med school. German med school takes six and a half years to teach the same amount of knowledge, and that's already a brutally challenging course of study. (At least it was to me, a reasonable driven straight-A student.)

Looking at the American med students I know, getting through med school in four years seems to require giving up all semblance of a private life and driving yourself to the edge of exhaustion/over the edge into a mental break-down. I don't think there's a way to do it in three years or even less.

That said, there might be a market for specialists with less training than that. There's no real reason to make e.g. an aspiring orthopedic surgeon sit through the entirety of med school, including all the dermatology/pediatrics/gynecology etc stuff.

But as another commenter mentioned, you'll have a hard time getting the regulatory bodies to accept your "eight months to orthopedic surgery" students as fully qualified doctors