Is it possible to get a PhD in physics in 3 years? If not, why? Almost every two weeks I get to know another PhD Student who needs much more time to obtain his/her PhD than he/she actually thought he/she would need. I would like to know if this is a specific Problem of my university/ my country or if this is completely normal. Besides that, I would like to know why you think that the PhD students actually need so much time to get their PhD. When talking about needing more time than planned, I mean that instead of needing 3 years they often need between 4-6 years.

Some observations that I have made on my own are:

-Often the PhD students are loaded with work that does not relate to their PhD-project. This means for example, that they have to do a lot of work for their professor, do some teaching, supervise undergradute students,... But I think the "worst" part here is that often they are loaded with work by their professors.
-The Project gets longer and longer. Even if they accomplish their project withing two years, the students will be told to do something new. This may even happen several times. 
- As a consequence from the last point that I mentioned a lot of the PhD students do not work as much as they would if they knew that they only have to finish their project and then they are finished. Since they know that even if they "finish" their work, they will get an additional task and won't be finished. Therfore, the motivation to finish the work as quickly as possible is rather low.

I also have the impression that in theoretical physics it is rather possible to obtain a PhD degree within 3 years, but not in experimental physics. I have recently heard that the average PhD student in physics needs 4.5 years. 

What are your experiences? Do you share my expressions or am I wrong? Is there a way to "make sure" at the beginning of your thesis that you won't need 5 years for it?

If there is the same problem in other areas of research, of course I would like to hear about this, too.
I have personally known only 2 people who got their PhD in 3 years in the US (though in Computer Science, not in Physics). I don't think it's impossible in Physics either, especially if you already have a masters going into the PhD, and if you have some research experience already so you can hit the ground running.

The problem generally with time to completion of a degree is simply that to do it in the near-minimum amount of time requires everything to go more or less according to plan, and you either have to get pretty lucky or just accept any outcome of your research efforts. I believe the root cause is simply that having a PhD where you have done the absolute minimum required, in the absolute minimum of time, has an uncertain (and perhaps limited) value. 

If you've learned all you really needed to learn, can demonstrate appropriate mastery, fulfill all program requirements, and have established the level of knowledge, skill, and experience required to move on to whatever you want to do after your PhD - fantastic! Based on the fact that average time to completion is higher than 3 years, this suggests that a large portion of people don't find that 3 years was enough for them.

There are myriad reasons for why time to completion of degrees is well beyond the minimum on average, but you could roughly condense them to: personal factors, adviser factors, and program factors. Some people just don't achieve the minimum to get approved for the PhD in 3 years, and some people just go way, way beyond the minimum before officially completing their program. What's most important is that you are clear - or get clear - on what you want to get out of your program, and what you want to do after you have completed it, and then plan appropriately to get the most out of the opportunity you have. That may mean a  completion time that is lower or higher than average, but what is most important is that you get what you came for in the first place.