Re: 3rd trimester?
I've heard anywhere from wk 27 - 29 depending on the source.
For your birth plan, I discussed mine w/my OB at 31 wks (last wk), and she didn't seem to mind. She put a copy of it in my medical file. I know I had mentioned having a birth plan to her at 23 wks and she told me to bring it in whenever I had a draft of it.
I haven't had a baby yet, so take this w/a grain of salt, but I would probably just bring in the birth plan at your next doctor's visit and discuss it then (if that's possible).