We_①_ been _②_ #Shuttle 31 years when it was _③_ in 2011. May 27 @ 5:33am Asia Pacific Time (4:33pm Eastern Standard Time) 2 @NASA astronauts launch on a @SpaceX #Falcon #CrewDragon to the #ISS. Watch @ http://nasa.gov.
Answers And Explanations
1. had (because been is present)
2. flying (because had+ been requires a gerund)
3. cancelled (because the past tense is necessary after was in this context)