If you believe celebrity deaths come in threes...
Then it would seem that Johnny Cash and John Ritter round out the trio led off by Warren Zevon.
I don't think Johnny Cash's passing is all that much of a suprise; he'd been in ill-health for a while, and he seemed like the sort of person who was unlikely to outlast June anyway. Still, knowing it's coming just gives you time to be prepared--it doesn't lessen the loss any.
Unfortunately, John Ritter really only serves as a source for jokes about, "Three's Company".