The "tradition" of celebrities dying in threes has been around for a while. I researched (well, googled) the topic and read that it's believed to have started when Buddy Holly, Richie Valens and Big Bopper died in a plane crash in 1959.
Some examples cited were that Heath Ledger, Suzanne Pleschette and Brad Renfro (?) all died the same week, as did Dennis Weaver, Darrin McGavin and Don Knotts. It appears there have been many other similar groupings depending on how you assess the celebrity status of the deceased or how long you stretch the period in which the deaths occur. (In that case, some might include David Carradine in the latest bunch.)
So, we can all rest easy now in the calm assurance that no more celebrities are likely to die in the near future.