3.6 means the firmware version of the blackberry. the most recent is 4.1, some people still run 4.0 or 3.6 though.
the RIM API integrates new functions from time to time.
Since: JDE 3.6.0 means that this class was integrated in the 3.6 firmware and can only be used on devices running 3.6 or higher.
i never encountered a device running a pre-3.6 version so i think you are on the save side