There is nothing wrong this stabilized 30Vdc power supply. Having not tested the orginal circuit, it must be a good design, or it would not be posted. Since it's a good design, but doesn't meet your requirements, a totally new design is needed. The engineering principle behind this design is found in many different power supplies. Can't you just buy this power supply from the website?
This power supply project is defective which is why I fixed it:
1) It is not reliable since many parts (the transformer, the rectifiers, the transistors and many resistors) are overloaded and burn out.
2) The opamps operate with a total supply voltage that is more than their maximum allowed supply voltage.
3) It does not provide 30V at 3A because the transformer voltage is too low.
This project is a kit sold by a company in Greece. I don't know what they do about people complaining about its problems.
Many defective projects have been posted here. I fixed a few of them.