3 Year Master's Full Time? How strange is it to want to spend three years on a Master's degree to take an extra six courses? A lot of programs have different curriculum for introductory PhD students vs introductory Master's. I want to be able to take some of those PhD classes, and I want to have spare room for exciting electives both within my Master's and outside it in associated fields.

When is this not possible, and does spending an extra year in a Master's program sound bad? I'd simply end up spending all my time  self-studying the extra material after a two year degree anyway.

Edit: I don't know how I'd feel applying for a PhD then dropping to a Master's in a couple years to have a separate core sequence. I don't want to waste anyone's time.

Edit: I'm not enrolled yet. Funding would hopefully be a combination of resident tuition and a TAship/RAship.
Many master's programs do have a term limit associated with them. However, many master's programs may require additional classes, particularly for students coming from other backgrounds. So you should definitely check the specific programs you're interested in enrolling in, to see what is specifically allowed.

But extending the master's or PhD program just to take classes seems like a poor use of resources. One of the goals of advanced education is learning to become an expert in a new subject. After a while, you should be able to get much of the same content that you would from enrolling in a class through self-study.