design 30V to 5V (up to 1.2A) convertor
what does D3 needed for ?


D3 prevents reverse voltages misapplied to the input destroying the regulator chip.
But this circuit won't be good enough for supplying anything more than a few tens of mA. You need to use a buck converter and not a linear regulator because linear regulators will get very hot in this application when supplying hundreds of mA (and you need amps!).
So basically I'm not going to labour over the other questions other than to say that the  P7805-2000-S is a more suitable choice but, it can only supply 2 amps: -