Most medical school curricula are 4 years because in 1910 the Flexner Report concluded that Johns Hopkins's 2+2 was the gold standard.
Per the LCME, an accredited MD program must have at least 130 instructional weeks. Since school starts in the fall and ends in the spring, a 3-year program is closer to 2.75 years in duration. The math works out to about 143 weeks from orientation to graduation, of which 130+ have to spent in school. That's a tight fit.
It's certainly doable, and the pace isn't necessarily faster than a 4-year program, but you won't get many breaks. There won't be room for much research, or clinical electives outside of core rotations. If you fall behind it may be difficult to remediate and stay with your cohort.
For those destined for primary care fields the model works well. There is no need for significant research time or a full complement of electives/sub-I's, and overall competition for residency slots is light.