After 3 years... Should I stay at a PhD position that I think is holding back my thesis?
The situation you are in is unsustainable. If you let it happen they will continue to abuse you forever. Evaluate the various people around you and separate those that are supportive from those that are not. If it is your academic advisor that is supportive then you might have a path in your current program, but if not, you need to find a way to exit. 

You don't give any evidence here that you are unsuitable for continued study, but you need to take a more proactive stance to realize your goals. 

Not every place is like what you are experiencing, nor is every boss or advisor an exploiter of students. 

But take stock of your options and don't be shy about moving on.