I need suggestions for a circuit to regulate the output of 24V (up to 28V) rectifier, the regulated voltage must be 24V with a maximum current of 3A and a nominal current of 1.2A , which is better to use switching converter or linear regulator?

 

Lm2956 chip

 

Lm2956

The Lm2956 need at least 1.5 volts of head room. At 24 volts out it needs at least 25.5 volts in.

What is you unregulated voltage at full load? Problem; at full load there is probably 2 volts of ripple. A meter will measure 26V but with ripple that voltage probably will be anywhere from 25 to 27. (could be more that 2V)
I think you have 28 volts with no load. (ripple voltage = 0)

 

Buck Boost
Try the link. I think it will do the job. It is only good to 30V so a little close.
It can increase or decrease the voltage. It can take in 23V and output 24V and/or input 25 and output 24.