I don't understand the way that the salary cap works in the NBA, but based upon what I've read it is my understanding that, as Rick said, all 3 of them would have been able to get paid more had they stayed and re-signed with their teams.
Of course there is no state income tax in Florida, so signing there offsets some amount of the difference for Bosh and James. How much I do not know. Wade was already in Florida.