The A parts that go on the cab sides are HALON fire extinguisher boxes and are included in Trumpeter’s kit as parts K7. The instructions only show one being added to the driver’s side in step 21. Then later in step 34, the passenger side has one too.
Part C is the external condenser to the ECU (Environmental Control Unit - A/C unit) and part B is an APU (Auxiliary Power Unit - Generator). A little more info on them.
Here are measurements I took for a fellow modeler of the A/C unit and HALON box.
You can also see where I used the same Dragon parts on my Trumpeter MLRS undergoing maintenance last year.