Why even ask about Pierce? He is a HOF player who played most of his career here and who won a final where he was finals MVP.
As far as the others go, KG is easy because he was so popular. Rondo is tougher. The Celtics have a really low bar for retiring numbers compared to, for example, the Lakers, so Rondo has a decent change since he started on a championship team and was an all star for the team for a few years.
Ray is a tougher call. He was never better than 3rd best player on a team that only one 1 title. In the 80s, did we have any other all star caliber players that only played 4-5 years with the team and only won 1 title as a supporting cast member? Our retired number starters those years were together for all 3 titles except for Maxwell, who played a lot longer with the team and was part of 2 titles.