Could you clarify your current position ? 4th year in med school or undergrad?
While I'm not in med school or in a graduate program. Here's my 2 cents:
PhD in 3 years is hard to do. It depends on your research topic / area [ assuming you're talking about a bio PhD ]. My mentor got his PhD in 3.5 years but then again he kept his nose to the grind stone and just pushed on through. That's considered super fast even by todays standards [ he got his PhD almost 30 years ago]. Some programs across the US has made it a standard to have their graduate students publish a first author paper before completing their PhD [ some former programs allowed students with 2nd author publications to graduate ].
Bottom line: you finish at YOUR pace. If you get placed on a good project and you can gather good data then perhaps you can finish faster assuming you don't end up trouble shooting a method for like 3 months etc.
Assuming you are a 4th year in med school and you've done research at your university during your medical school career. You might be able to get into the PhD program and finish it in 3 years.
I've seen a few people get their PhD in 3 years because they've worked in their lab since undergrad. That time spent in undergrad gave them a clear focus and project [lay of the land ] so that when they started their graduate career they just continued on with the project or another aspect of their project from undergrad and finished really fast.
If you have a similar case in med school i can't see why you can't finish a graduate school in a timely fashion assuming you know what type of project you want to do, lab to join etc.
Here's something else to think about:
Look at your faculty directory. How many of those people have a PhD? How many people have a MD ? How many people have a MD/PhD?
The trend is that a lot of people have PhDs and MDs but very little people have MD/PhD.
Bottom line: you don't need a PhD to do research.
*Please note this is what I have noticed from my university.
Last but not least where do you see yourself in 10 years?
Do you see yourself in a lab ? Treating patients ? or both ?
Both would be ideal but do you think you can handle getting a faculty position and eventually tenure / get grants / see patients / have a life & familiy etc
Don't get me wrong I love research and I love medicine. And I think the MD/PhD is a great program but I am a bit harsh at times.