The three of us haven't gotten together since we graduated.
We haven't been together since we finished school.
ーThe three of us haven't gotten together since we graduated.
「私たち3人は卒業して以来集まっていない。」
get together で「集まる」
since we graduated で「卒業して以来」
ーWe haven't been together since we finished school.
「私たちは学校を卒業して以来、一緒になっていない。」
be together で「一緒にいる・なる」
since we finished school で「学校を卒業して以来」
ご参考まで!