Assuming that the circuit works, which I am not convinced it does!
(What on earth is T3 supposed to be doing?)
The voltage at the non-inverting input of the op-amp is set at 3.6v by the zener.
The op-amp will drive its output to try and make the inverting input the same voltage as the non-inverting input.
The voltage at the inverting input is derived from Vout by the potential divider formed from the 8k and 10k(variable) resistors.
So Vinv/Vout = Rvariable/(Rvariable + 8K)
and
Vinv is 3.6v when the circuit is working correctly.
Now you re-arrange the equations and calculate Vout.
The output current is probably limited to appoximately 1.5 amps due to T2 and 0.4 ohm resistor.
I'm a little puzzled however at the 8k and 10k pot, which seems to allow adjusting only down to around 6.5 volts and the schematic shows a 3v to 7 volt Vcc. Maybe this circuit was designed to limit the max output voltage.
T3 could be a fast voltage clamp, used to bypass the slower op amp which takes much more time to respond to a fast change in Vcc. The only strange thing there is that the op amp will also take time to come out of saturation if that bypass is invoked for a long enough time period. I guess it could also protect against a zener blow out condition.
Where did this circuit come from and what was its original intended purpose?
Well this circuit is a little strange in that if you set the pot to 50 percent which is 5k the output would want to go to about 9.36 volts, but the problem is there isnt enough Vcc to supply that high. Could the original supply have been a 15v supply?
I think you got it! Also, as audioguru points out, T3 may be being used for a temperature cut out sensor. As the Vbe voltage requirement lowers with temperature increase, the transistor 'may' draw more current through the collector, although this action is highly dependent on the actual transistor part number.
T3 is just a safety if the Zener breaks down, will pass more voltage .. this tension will most excite the base of T3, so the collector voltage will create a point of difference with the sender .. and protects the circuit to not pass a high value for the output!