That circuit has been at
www.electronics-lab.com for at least 11 years and was a kit from Greece.
It is not reliable because many of its parts are overloaded, for example its TL081 opamps have a maximum allowed total supply of only 36V but 2 of them operate with more.
It originally used an overloaded 24VAC transformer but the rectified and filtered voltage is too low for a regulated output of 30V at 3A.
A few of us fixed it using modern opamps that have a max total supply of 44V and a 28V or 30V transformer that can provide enough current. The main filter capacitor value was increased a lot. We added calibration trimpots.
The output transistor on the original circuit got so hot it melted so our fixed version uses two output transistors to share the heat.
I did not look at the datasheets for the Oriental transistors used in the Chinese kit to see if they are suitable.