Some of the phased out Mk.3s have been sent to reserve units to replace Mk.2s; some will be converted to Ofek HAPCs (production series vehicles are Mk.3 based as opposed to the prototype which was converted from a Mk.2); some, I guess, will replace specialized vehicles such as the ancient, Magach based AVBL and the Sho’t based Nagmachon and Nakpadon.
I vaguely remember reading somewhere that all Namers rolling off the production line these days are Mk.4 based, however, I might be wrong about that.