Very few exist, from what I heard, the LCME doesn't encourage schools offering a 3 year MD. Exceptions do exist, I think NYU Med has some 3 year integrated MD track.
Hard but also only good if you know which specialty you want to go into.
Some 3 year programs only let you go into primary care, while other(s) you can go into any one. Some you have to be accepted to the 4 year program before you can apply into the 3 year.
UC Davis accelerated primary care education is a 3 year MD with conditional acceptance into internal med and family med through Kaiser and another affiliated hospital. They want to expand to also include general surgery and pediatrics