See how there is a total for Alpha, Beta, Gamma for Income/Expenses/P&L - is there a way to then be able add these three columns up to give you a total P&L?
First, the Guide you linked to is not about custom reports. It is about tracking codes. Those are very different things.
Second, for a combined P&L, simply add a column for which you leave the Tracking Code field blank. You will get the entire company. Strictly speaking, this is not the sum of the other three columns; they are all subsets of the whole company.