Agree--do PM&R third year. Early experience is much better. Plenty of PM&R applicants don't do their first PM&R rotation till 4th year, but doing it early gives you the chance to confirm what you want to do, and leaves those early 4th year electives open for audition rotations should you decide on PM&R.
Plus, while rare, I've heard of plenty of people wanting to do their first choice specialty last so they look as good as possible, but then decide they don't actually like the specialty. Unless you also happened to love one of the core-rotations, this can put you in a really poor position if you only have ~2 months to decide on a new specialty.