Who to include as a reference? I am finishing my Ph.D. and looking for a postdoc position.
Right now I have included my current supervisor in Ph.D. and my previous supervisor from my masters as references.
The postdoc position advertisement does not state the number of references required, only a reference list.
However, I have mostly read online and by checking other CV's that including three references is the norm when you are a fresh Ph.D. graduate.

I was considering asking a professor who I had for a class during my Ph.D. to be my third reference. We have had some discussions about my research, and we have good friendly relations. 
I was also considering asking a postdoc researcher that I have worked with in the past but who is not a tenure track faculty yet.

Is it a good idea to include people that you have not worked with directly on your research as references? Or people that are still postdocs?
Who can be my potential reference aside from my supervisors? 

Your help would be much appreciated.
For letters of recommendation, it's useful to have them written by people with a strong reputation, and by people who have seen you work. These do not need to be the same people. I would base your considerations off how much of this is already covered by your Masters supervisor and your PhD supervisor. There's nothing wrong with getting a letter of recommendation from a postdoc, or from someone you haven't worked with directly. In that vein, may I recommend your department head as a possible reference, if they are acquainted with your work?

Finally, I'd like to mention that I've never heard of three letters of recommendation being better than two. This is in Europe, your mileage in other parts of the world (or different parts of Europe) may vary.