Am I graduate student or underpaid employee? I started graduate course in a national research institute abroad (3 years ago). I just realized now this is my third year and every year we had different projects. 

Main problem is that I am not having enough time to focus in one area to publish papers.  Second problem is that I am not obtaining knowledge from the other doctors.

Should I stop pursuing PhD if I feel I am not learning as much as I expected and feeling I am doing someone else work and tasks?
The downside of academia that you are describing definitely exists.  Have been hearing this for generations now (don't get me started on the pyramid, tournament style rewards, job prospects, etc.).
All that said, it is irrelevant to you.  Do NOT cut your nose off to spite yourself.  You are too close now.  Just buckle down and finish and get the degree.  It is helpful. And you will kick yourself if you miss it.
Be a little selfish about taking care of yourself on papers, etc.  Focus, focus, focus.  Pick your spots,  but I would avoid volunteering always and also occasionally push back on collateral duties.  And push, push, push for papers.  They really are the score card...don't let anyone kid you the opposite.