"3年前公演をするはずだった。"
- It was supposed to be held three years ago.
オペラの公演が中止になってしまいました。3年前公演をするはずだった
- The opera was supposed to be held three years ago.
- They were supposed to perform the opera three years ago.
- It was supposed to be performed three years ago.
ご質問ありがとうございます。
「3年前公演をするはずだった」は英語で「It was originally meant to be held 3 years ago.」と言います。
もし、「オペラの[公演](https://eikaiwa.dmm.com/uknow/questions/43217/)」を指したいなら、「The opera was originally supposed to be held 3 years ago.」になります。
ご参考になれば幸いです。