Who to include as a reference?
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.