It's actually more of a voltage stabilizer. Drawn in more conventional way, it is an emitter follower with the base voltage held constant by the Zener. All R1 and the three diodes do is limit the output current. It isn't constant current, it's constant voltage with current limiting, almost the opposite of constant current.
Under light load, the base would be at 18V, the emitter at about 17.3V so the current is limited to at most 17.3mA but D6 further limits it to about 16.7mA. However D3 holds the output from rising above 12V and the chain of diodes pulls the base down to about 15.5V which further reduces the available current. It's a strange circuit and I can't see what it is supposed to do except possibly keep the current through D3 more stable to help it hold constant voltage. If that is the case, using a 3W device for D3 seems inappropriate.
Brian.