PhD in 3 years? What is the shortest span, you can do PhD?

A full time PhD programs in the UK and rest of Europe take around 3 to 4 years to complete. However, it is highly unusual to finish a PhD in even 3 years, but it is possible. Well, it depends on your area of study-it's impossible to finish Medical PhDs in 2 years, physics PhDs typically take 5 years, humanities PhDs tend to take longer than science or engineering.
It also depends on type of PhD, type of your research, and various personal factors and most importantly, on your supervisors, feedback type and time.

You might be able to finish your your PhD in two years, but if your goal is to finish a PhD in two years, IMO you shouldn't probably be doing one.

Getting a PhD is not about gaining a degree in the shortest time possible, rather it is the time of your life where you work on what excites you, meet a lot of like-minded people, build innovative solutions, give a lot of talks and importantly become a noted expert in your field.

This is something that takes more than 2 years and one should make the best use if it. I can't think of any place that would pay you to do what you want with complete independence, isn't?