Can I use text from my dissertation in a manuscript? There has been general discussion of whether one can use a published work in their dissertation, with the consensus being 'of course'! The first two chapters of my thesis will be published works. I'm interested in the reverse now; I intend to write the third chapter of my thesis as a draft of sorts for a publication. Now I'm wondering whether including text and figures from my dissertation in a [future] manuscript constitutes 'self-plagarism'.

A fellow graduate student advised me that it's only 'self-plagarism' if I formally copyright my thesis, however, I'm skeptical of that being the important distinction.
In general this is OK or, in some fields, even recommended. It does not make any difference if the dissertation comes first and then the publication or vice versa.

However, I would reference the thesis/paper(something along the line: this paper was part of my dissertation ... or this chapter was published as ...) and you should talk to someone (your supervisor? the postgrad coordinator?) in your department about that. There might be some rules regarding that practice (I never heard of anything like that but just to be on the safe side).

Btw: Self-plagarism has nothing to do with copyright violations.