We're not talking Hall of Fame here.. If a player means alot to the organization his number should be retired. I don't think you necessarily need a criteria to have a team retire your number. With that said, KG and Ray played 5 years with the C's, all-star appearances, 1 title. That's good enough in my book.
Reggie was named captain during his final year, which was a great honor at the time. He was always one of the top Celtics during his time, increased his scoring every year in the play-offs and came up big in the clutch. Like the big 3, he could have had better numbers had he not played with the original Big 3.