Im a little confused there is techinically no 767-300 3 class plane that is used for domestic flights. Of course there are many exceptions, as there are with the international 777. As it is now the UA 767 fleet, to the best of my knowledge is as follows.

767-300ER - International plane with no FS but with C+

767-300 - 2 class domestic plane that is just a twin aisled 757 or A320 in terms of comfort

767-200 This is the dreaded 3 class transcon plane that I suspect you are talking about. I think pitch is similar to International C class but the seats are much older.