This is the 17657th most frequent Korean word.
셋이
"셋이" means "three people" or "the three of us."
The word '셋이' is used to indicate the group of three people performing the action together.
우리는 셋이 함께 영화를 보러 갔어요.
The three of us went to watch a movie together.
Here, '셋이' refers to the group of three friends all deciding on the same action.
친구 셋이 다 같이 식당에 가기로 했어요.
Three friends decided to go to the restaurant together.
In this context, '셋이서' adds '서' to emphasise that the group of three performed the action entirely amongst themselves.