Should one continue to say all Shabbos additions to Birkas Hamazon during a Seudas Shlishis, if the time ending Shabbos has passed?
Yes one should continue bentching even though shabbos has already passed. This is because the meal started when it was still shabbos.
There is an interesting issue, what one should do when Rosh Chodesh is on Sunday. Then we have a double issue; if he says ritzay he is saying that today is still part of shabbos, because it really is Rosh Chodesh, if he says yaaleh v’yavo, he is essentially saying that it is now Sunday. There are varying opinions regarding this, and therefore it is preferable to eat any bread before sunset to avoid this issue. In your case however the is no problem at all.
O:CH 188-10 M:B 10,33 Shmiras Shabbos Khilchoso 57- 13b in the name of R’ S.Z. Auerbach zt”l